Summary Prospectus    May 1, 2023

Emerging Markets Debt Fund

Before you invest, you may want to review the Fund’s prospectus, which contains more information about the Fund and its risks. You can find the Fund’s prospectus, statement of additional information and other information about the Fund online at williamblairfunds.com/prospectus. You can also get this information at no cost by calling 1-800-742-7272 or by sending an e-mail request to fundinfo@williamblair.com. The Fund’s prospectus and statement of additional information, each dated May 1, 2023, as supplemented, are incorporated by reference into this Summary Prospectus.

INVESTMENT OBJECTIVE:    The William Blair Emerging Markets Debt Fund seeks to provide attractive risk-adjusted returns relative to the Fund’s benchmark.

FEES AND EXPENSES:    This table describes the fees and expenses that you may pay if you buy, hold and sell shares of the Fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.

Shareholder Fees (fees paid directly from your investment):

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment):

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Example:    This example is intended to help you compare the cost of investing in shares of the Fund with the cost of investing in other mutual funds. The example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year and the Fund’s operating expenses remain the same. The figures reflect the expense limitation for the first year. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Portfolio Turnover:    The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 118% of the average value of its portfolio.

PRINCIPAL INVESTMENT STRATEGIES:    Under normal circumstances, the Fund invests at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in debt instruments that are economically tied to emerging market countries, which may be represented by forwards or derivatives such as options, futures contracts or swap agreements. Emerging markets include every country in the world except countries included in the MSCI World Index.

Debt securities in which the Fund invests include fixed-rate and floating-rate bonds issued by various public (governmental) and private (corporate) issuers (including private placements and restricted securities). The Fund invests a significant portion of its assets in sovereign debt securities (debt securities issued or guaranteed by foreign sovereign governments or their agencies, authorities or political subdivisions or instrumentalities, and supranational agencies) and debt securities of quasi-sovereign issuers (entities owned by a sovereign government). The Fund may also invest in debt securities issued or guaranteed by foreign corporations and foreign financial institutions. The Fund may invest in debt securities of any credit rating, including investment grade securities, below investment grade securities (commonly referred to as “high yield” or “junk bonds”), and unrated securities. The Fund may invest up to a maximum of 10% of the Fund’s net assets in distressed or defaulted debt securities. The Fund may invest in securities of any maturity.

The majority of the Fund’s investments are generally made in securities of issuers based in South and Central America (including the Caribbean), Central Europe, Eastern Europe, Asia, Africa and the Middle East. More specifically, the Fund invests in countries where the Adviser is able to assess the specific political and economic risks and in countries that have undertaken certain economic reforms and reached certain growth objectives. Countries in which the Fund invests include frontier markets (emerging markets that are early in their development).

The Fund invests in assets denominated in the currencies of economically developed and politically stable countries that are members of the Organisation for Economic Co-operation and Development (OECD), as well as in assets denominated in local currency. The Adviser may, but is not required to, hedge the currency risk associated with the Fund’s investments.

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As part of its investment strategy, the Fund may utilize derivatives, including futures and forward contracts, swaps (including credit default swaps and total return swaps), credit derivatives, and currency-related derivatives. Derivatives are primarily utilized to hedge interest rate duration risk and foreign exchange risk.

Additionally, the Fund may maintain assets in cash, deposit, call or current accounts or invest in short-term instruments, such as money market funds, U.S. or other government securities, certificates of deposit, bankers’ acceptances or similar temporary investments, to meet the expense needs of the Fund and/or to fund withdrawals or for such other purposes as may be determined by the Adviser.

The Fund is measured against the JPMorgan Emerging Markets Bond Index (EMBI) Global Diversified as its primary index. The Fund is actively managed within its objective and is not constrained by a benchmark.

THE FUND IS NON-DIVERSIFIED UNDER THE INVESTMENT COMPANY ACT OF 1940, AS AMENDED, AND MAY INVEST A LARGER PERCENTAGE OF ITS ASSETS IN FEWER ISSUERS THAN DIVERSIFIED MUTUAL FUNDS.

PRINCIPAL RISKS:    The Fund’s returns will vary, and you could lose money by investing in the Fund. The following is a summary of the principal risks associated with an investment in the Fund.

The Fund involves a high level of risk and may not be appropriate for everyone.    There can be no assurance that the Fund’s investment objective will be achieved. The Fund is not intended to be a complete investment program. The Fund is designed for long-term investors.

The principal risks of investing in the Fund (in alphabetical order after the first six risks) are:

Market Risk.    The value of the Fund’s investments may go up or down, sometimes rapidly or unpredictably. The value of an investment may decline due to factors affecting securities markets generally or particular industries represented in the securities markets. The value of an investment may decline due to general market conditions that are not specifically related to a particular company, such as real or perceived adverse economic conditions, recessions, inflation, rapid interest rate changes, changes in the general outlook for corporate earnings, changes in interest or currency rates or adverse investor sentiment generally. The value of an investment may also decline due to factors that affect a particular industry or industries, such as labor shortages or increased production costs and competitive conditions within an industry. During a general downturn in the securities markets, multiple asset classes may decline in value simultaneously. Geopolitical and other events may also disrupt securities markets and adversely affect global economies and markets and thereby decrease the value of the Fund’s investments.

Credit Risk.    The value of the Fund’s fixed income securities is subject to the ability of the issuers of such securities to make interest payments or principal payment at maturity. The credit ratings of issuers could change and negatively affect the Fund’s share price or yield. The Fund’s net asset value and total return may be adversely affected by the inability of the issuers of the Fund’s securities to make interest payments or payment at maturity. The Fund’s investments in obligations issued or guaranteed by U.S. Government agencies or instrumentalities may not be backed by the full faith and credit of the United States and may differ in the degree of support provided by the U.S. Government.

Foreign Investment Risk.    The risks of foreign investments may include less publicly available information, less stringent investor protections and disclosure standards, less governmental regulation and supervision of foreign stock exchanges, brokers and issuers, share registration and custody, less stringent or a lack of uniform accounting, auditing and financial reporting standards, practices and requirements, the possibility of expropriation, seizure or nationalization, confiscatory taxation, limits on repatriation, adverse changes in investment or exchange control regulations, political instability,

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restrictions on the flow of international capital, imposition of foreign withholding or other taxes, fluctuating currencies, inflation, difficulty in obtaining and enforcing judgments against foreign entities or other adverse political, social or diplomatic developments that could affect the Fund’s investments. Foreign investments may be less liquid and their prices more volatile than the securities of U.S. companies. The Fund is expected to incur operating expenses that are higher than those of mutual funds investing exclusively in U.S. securities due to the higher custodial fees associated with foreign securities investments.

Emerging Markets Risk.    Foreign investment risk is typically magnified in emerging markets, which are the less developed and developing nations. These risks are further magnified in frontier markets, which are among the smallest and least mature investment markets.

Sovereign and Quasi-Sovereign Default Risk.    The Fund invests in securities issued by or guaranteed by non-U.S. sovereign governments (known as sovereign debt securities) and in securities issued by entities that are owned or guaranteed by non-U.S. sovereign governments (known as quasi-sovereign debt securities). An issuer of sovereign or quasi-sovereign debt held by the Fund, or the governmental authorities that control the repayment of the debt, may be unable or unwilling to repay the principal or interest when due. This may result from political or social factors, the general economic environment of a country or levels of foreign debt or foreign currency exchange rates.

Interest Rate Risk.    Normally, the values of fixed income securities vary inversely with changes in prevailing interest rates. The value of the Fund’s fixed income securities tends to decrease when interest rates rise and tends to increase when interest rates fall. Securities with longer maturities or durations held by the Fund are generally more sensitive to interest rate changes. As such, securities with longer maturities or durations are usually more volatile than those with shorter maturities or durations. The negative impact on fixed income securities from rising interest rates could be swift and significant and negatively impact the Fund’s net asset value. Risks associated with rising interest rates are currently heightened because the U.S. Federal Reserve has raised, and may continue to raise, interest rates. A rising interest rate environment may also result in periods of increased redemptions from fixed income funds and increased supply in the market due to selling activity to meet redemptions. If the Fund has to liquidate portfolio securities to meet redemptions in such an environment, it may have to do so at disadvantageous times and prices, which could negatively impact the Fund’s net asset value.

Aggressive Investment Technique Risk.    The Fund may use investment techniques and financial instruments that may be considered aggressive, including but not limited to the use of futures contracts, options on futures contracts, securities and indices, forward contracts, swap agreements and similar instruments. Such techniques may also include taking short positions or using other techniques that are intended to provide inverse exposure to a particular market or other asset class, as well as leverage, which can expose the Fund to potentially dramatic losses or gains. These techniques may expose the Fund to potentially dramatic losses in the value of certain of its portfolio holdings.

Counterparty and Contractual Default Risk.    The Fund’s investments in derivatives and other financial instruments that involve counterparties subject the Fund to the risk that the counterparty could default on its obligations under the agreement, either through the counterparty’s failure or inability to perform its obligations or bankruptcy.

Credit Default Swap Risk.    Credit default swaps are subject to the credit risk of the underlying reference obligation and to counterparty credit risk. If the counterparty fails to meet its obligations, the Fund may lose money. Credit default swaps are also subject to the risk that the Adviser will not properly assess the risk of the underlying reference obligation. If the Fund is selling credit protection, there is a risk that a credit event will occur and that the Fund will have to pay the counterparty. If the Fund is buying credit

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protection, there is a risk that no credit event will occur and the Fund will receive no benefit for the premium paid. Credit default swaps may be difficult to value and may have the effect of leverage on the Fund.

Currency Risk.    The value of the Fund’s portfolio may be affected by changes in exchange rates or control regulations. If a non-U.S. currency gains against the U.S. dollar, the value of the security increases in U.S. dollar terms. If a non-U.S. currency declines against the U.S. dollar, the value of the security decreases in U.S. dollar terms.

Derivatives Risk.    Investing in derivatives, including futures and forward contracts, swaps, credit derivatives, and currency-related derivatives, involves investment techniques and risks different from those associated with ordinary mutual fund securities transactions and may involve increased transaction costs. The Fund’s investment in derivatives may rise or fall more rapidly in value than other investments and may reduce the Fund’s returns. Changes in the value of the derivative may not correlate perfectly, or at all, with the underlying asset, reference rate or index, and the Fund could lose more than the principal amount invested. Derivatives also may be subject to certain other risks such as leveraging risk, liquidity risk, interest rate risk, market risk, credit risk, counterparty risk, operational risk, legal risk, management risk and the risk of mispricing or improper valuation. Derivatives also may not behave as anticipated by the Fund, especially in abnormal market conditions. The use of derivatives may increase the volatility of the Fund’s net asset value. Derivatives may be leveraged such that a small investment in derivative instruments can have a significant impact on the Fund’s exposure to stock market values, interest rates, currency exchange rates or other investments. As a result, a relatively small price movement in a derivatives contract may cause an immediate and substantial loss or gain. It may be difficult or impossible for the Fund to purchase or sell certain derivatives in sufficient amounts to achieve the desired level of exposure, which may result in a loss or may be costly to the Fund. In addition, the possible lack of a liquid secondary market for certain derivatives and the resulting inability of the Fund to sell or otherwise close out a derivatives position could expose the Fund to losses and could make such derivatives more difficult for the Fund to value accurately. Some derivatives are more sensitive to market price fluctuations and to interest rate changes than other investments. The Fund also could suffer losses related to its derivatives positions as a result of unanticipated market movements, which losses are potentially unlimited. The Fund also may be exposed to losses if the counterparty in the transaction does not fulfill its contractual obligation. In addition, derivatives traded over-the-counter (“OTC derivatives”) do not benefit from the protections provided by exchanges in the event that a counterparty is unable to fulfill its contractual obligation. Such OTC derivatives therefore involve greater counterparty and credit risk and may be more difficult to value than exchange-traded derivatives. When a derivative is used as a hedge against a position that the Fund holds, any loss generated by the derivative should generally be offset by gains on the hedged instrument, and vice versa. While hedging can reduce or eliminate losses, it also can reduce or eliminate gains. Hedges are sometimes subject to imperfect matching between the derivative and the hedged investment, and there can be no assurance that the Fund’s hedging transactions will be effective. Also, suitable derivative transactions may not be available in all circumstances. Derivatives are subject to fees and other costs which are not reflected in the Annual Fund Operating Expenses table.

Derivatives are also subject to liquidity risk. Liquidity risk is the risk that a derivative instrument cannot be sold, closed out or replaced quickly at or very close to its fundamental value. Generally, exchange-traded derivatives are very liquid because the exchange clearinghouse is the counterparty of every contract. OTC derivatives are less liquid than exchange-traded derivatives since they often can be closed out only with the other party to the transaction. The Fund’s ability to sell or close out a position in an instrument prior to expiration or maturity depends on the existence of a liquid secondary market or, in the absence of such a market, the ability and willingness of the counterparty to enter into a transaction closing out the position. Therefore, there is no assurance that any derivatives position can be sold or closed out at a time and price that is favorable to the Fund.

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Distressed Debt Risk.    When the Fund invests in obligations of financially troubled companies (sometimes known as “distressed” securities), there exists the risk that the transaction involving such debt obligations will be unsuccessful, take considerable time or will result in a distribution of cash or a new security or obligation in exchange for the stressed and distressed debt obligations, the value of which may be less than the Fund’s purchase price of such debt obligations. Furthermore, if an anticipated transaction does not occur, the Fund may be required to sell its investment at a loss or hold its investment pending bankruptcy proceedings in the event the issuer files for bankruptcy.

Floating and Variable Rate Securities Risk.    For floating and variable rate securities, there may be a lag between an actual change in the underlying interest rate benchmark and the reset time for an interest payment of such a security, which could harm or benefit the Fund, depending on the interest rate environment or other circumstances. In a rising interest rate environment, for example, a floating or variable rate security that does not reset immediately would prevent the Fund from taking full advantage of rising interest rates in a timely manner. However, in a declining interest rate environment, the Fund may benefit from a lag due to a security’s interest rate payment not being immediately impacted by a decline in interest rates.

Certain floating and variable rate securities have an interest rate floor feature, which prevents the interest rate payable by the security from dropping below a specified level as compared to a reference interest rate (the “reference rate”). Such a floor protects the Fund from losses resulting from a decrease in the reference rate below the specified level. However, if the reference rate is below the floor, there will be a lag between a rise in the reference rate and a rise in the interest rate payable by the security, and the Fund may not benefit from increasing interest rates for a significant amount of time.

Geographic Risk.    To the extent that the Fund invests a significant portion of its assets in any one country or geographic region, the Fund will be subject to greater risk of loss or volatility than if the Fund always maintained wide geographic diversity among the countries and geographic regions in which it invests. Investing in any one country or geographic region makes the Fund more vulnerable to the risks of adverse securities markets, exchange rates and social, political, regulatory and economic events in that one country or geographic region.

High Yield Securities Risk.    The Fund invests in instruments including junk bonds and instruments that may be issued by companies that are highly leveraged, less creditworthy or financially distressed. These investments are considered to be speculative and are subject to greater risk of loss, greater sensitivity to economic changes, valuation difficulties and potential illiquidity. Such investments are subject to additional risks including subordination to other creditors, no collateral or limited rights in collateral, lack of a regular trading market, extended settlement periods, liquidity risks, prepayment risks, and lack of publicly available information. Because some instruments may have a more limited secondary market, liquidity and valuation risk is more pronounced for the Fund than for funds that invest primarily in other types of fixed income instruments or equity securities.

Income Risk.    The income received by the Fund may decrease as a result of a decline in interest rates.

Leverage Risk.    The Fund’s investments in derivatives or exposure to derivatives through other investment vehicles expose the Fund to leverage inherent in such instruments. Such leveraged investments can amplify the effects of market volatility on the Fund’s net asset value (i.e., relatively small market movements may result in large changes in the Fund’s net asset value) and make the Fund’s returns more volatile. At times, the Fund’s leveraged investments may cause the Fund’s investment exposure to exceed its net assets and could cause the Fund to experience substantial losses, including the risk of total loss, if the market moves against the Fund. The use of leveraged investments may also cause the Fund to liquidate portfolio positions when it would not be advantageous to do so in order to satisfy its obligations. The use of leveraged investments may also cause the Fund to have higher expenses than those of mutual funds that do not use such techniques.

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Liquidity Risk.    Investments that trade less frequently can be more difficult or more costly to buy, or to sell, than more liquid or active investments. It may not be possible to sell or otherwise dispose of illiquid securities both at the price and within a time period deemed desirable by the Fund.

Non-Diversification Risk.    The Fund is non-diversified, meaning that it is permitted to invest a larger percentage of its assets in fewer issuers than diversified mutual funds. Thus, the Fund may be more susceptible to adverse developments affecting any single issuer held in its portfolio, and may be more susceptible to greater losses because of these developments.

Operational and Technology Risk.    Cyber-attacks, disruptions, or failures that affect the Fund’s service providers, counterparties, market participants, or issuers of securities held by the Fund may adversely affect the Fund and its shareholders, including by causing losses for the Fund or impairing Fund operations.

Private Placement Risk.    Investments in private placements may be difficult to sell at the time and at the price desired by the Fund; companies making private placements may make less information available than publicly offered companies; and privately placed securities are more difficult to value than publicly traded securities. These factors may have a negative effect on the performance of the Fund. Securities acquired through private placements are not registered for resale in the general securities market and may be classified as illiquid.

Regulatory Risk.    Future regulatory developments could impact the Fund’s ability to invest in certain derivatives. It is possible that government regulation of various types of derivative instruments, including futures, options and swap agreements, may limit or prevent the Fund from using such instruments as a part of its investment strategies, and could ultimately prevent the Fund from being able to achieve its investment objective. It is impossible to predict the effects of future legislation and regulation in this area, but the effects could be substantial and adverse. It is possible that legislative and regulatory activity could limit or restrict the ability of the Fund to use certain derivatives as a part of its investment strategies and could alter, perhaps to a material extent, the nature of an investment in the Fund or the ability of the Fund to continue to implement its investment strategies.

The futures, options and swaps markets are subject to comprehensive statutes, regulations, and margin requirements. In addition, the U.S. Securities and Exchange Commission, Commodity Futures Trading Commission and the exchanges are authorized to take extraordinary actions in the event of a market emergency, including, for example, the implementation or reduction of speculative position limits, the implementation of higher margin requirements, the establishment of daily price limits and the suspension of trading. The regulation of futures, options and swaps transactions in the United States is a changing area of law and is subject to modification by government and judicial action.

Share Ownership Concentration Risk.    To the extent that a significant portion of the Fund’s shares is held by a limited number of shareholders or their affiliates, there is a risk that the share trading activities of these shareholders could disrupt the Fund’s investment strategies, which could have adverse consequences for the Fund and other shareholders (e.g., by requiring the Fund to sell investments at inopportune times or causing the Fund to maintain larger-than-expected cash positions pending acquisition of investments). In addition, separate accounts managed by the Adviser may invest in the Fund and, therefore, the Adviser at times may have discretionary authority over a significant portion of the assets of the Fund. In such instances, the Adviser’s decision to make changes to or rebalance its clients’ allocations in the separate accounts may substantially impact the Fund’s performance.

FUND PERFORMANCE HISTORY:    The information below provides some indication of the risks of investing in the Fund by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual total returns for the periods indicated compare with those of a broad

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measure of market performance. The Fund’s past performance (before and after taxes) does not necessarily indicate how it will perform in the future. For more recent performance information, go to www.williamblairfunds.com or call 1-800-635-2886.

Annual Total Returns.    The bar chart below provides an illustration of the Fund’s Class I shares performance over the past calendar year.

		Highest Quarterly Return 9.81% (4Q22)		Lowest Quarterly Return (13.36)% (2Q22)

Average Annual Total Returns (For the periods ended December 31, 2022).    The table below shows returns on a before-tax and after-tax basis for Class I shares and on a before-tax basis for Class R6 shares. After-tax returns for Class R6 shares will vary. After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. In some instances, the “Return After Taxes on Distributions and Sale of Fund Shares” may be greater than the “Return Before Taxes” because the investor is assumed to be able to use the capital loss on the sale of Fund shares to offset other taxable capital gains. Actual after-tax returns depend on an investor’s tax situation and may differ from those shown. After-tax returns are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.

MANAGEMENT:

Investment Adviser.    William Blair Investment Management, LLC is the investment adviser of the Fund.

Portfolio Managers.    Marcelo Assalin, a Partner of the Adviser, and Marco Ruijer, an Associate of the Adviser, co-manage the Fund. Messrs. Assalin and Ruijer have co-managed the Fund since its inception in 2021.

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PURCHASE AND SALE OF FUND SHARES:

Class I Share Purchase.    The minimum initial investment for an account generally is $500,000 (or any lesser amount if, in William Blair’s opinion, the investor has adequate intent and availability of funds to reach a future level of investment of $500,000). There is no minimum for subsequent purchases. There is no minimum initial investment for qualified retirement plans, including, but not limited to, 401(k) plans, 457 plans, employer-sponsored 403(b) plans, defined benefit plans and other similar accounts, or plans whereby Class I shares are held through omnibus accounts (either at the plan level or the level of the plan administrator) and certain other accounts. William Blair may make certain additional exceptions to the minimum initial investment amount in its discretion. Class I shares are only available to certain investors. See “Your Account—Class I Shares” for additional information on the eligibility requirements and investment minimums applicable to purchasing Class I shares.

Class R6 Share Purchase.    The minimum initial investment for an account is $1 million (or any lesser amount if, in William Blair’s opinion, the investor has adequate intent and availability of funds to reach a future level of investment of $1 million). There is no minimum for subsequent purchases. There is no minimum initial investment for qualified retirement plans, including, but not limited to, 401(k) plans, 457 plans, employer-sponsored 403(b) plans, defined benefit plans and other similar accounts, or plans whereby Class R6 shares are held through omnibus accounts (either at the plan level or the level of the plan administrator) and certain other accounts. William Blair may make certain additional exceptions to the minimum initial investment amount in its discretion. Class R6 shares are only available to certain investors. See “Your Account—Class R6 Shares” for additional information on eligibility requirements and investment minimums applicable to purchasing Class R6 shares.

Sale.    Shares of the Fund are redeemable on any day the New York Stock Exchange is open for business by mail, wire or telephone, depending on the elections you make in the account application.

TAX INFORMATION:    The Fund intends to make distributions that may be taxed as ordinary income or capital gains, unless you are investing through a tax-advantaged investment plan. If you are investing through a tax-advantaged investment plan, withdrawals from the tax-advantaged investment plan may be subject to taxes.

PAYMENTS TO BROKER-DEALERS AND OTHER FINANCIAL INTERMEDIARIES:    If you purchase shares of the Fund through a broker-dealer or other financial intermediary (such as a bank), the Fund and its related companies may pay the intermediary for the sale of shares and related services. These payments may create a conflict of interest by influencing the broker-dealer or other intermediary and your salesperson to recommend the Fund over another investment. Ask your salesperson or visit your financial intermediary’s website for more information.

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