MainStay MacKay Emerging Markets Debt Fund (Prospectus Summary) | MainStay MacKay Emerging Markets Debt Fund

MainStay MacKay Emerging Markets Debt Fund(formerly known as MainStay Emerging Markets Debt Fund)

Investment Objective

The Fund seeks current income. Capital appreciation is a secondary objective.

Fees and Expenses of the Fund

The table below describes the fees and expenses that you may pay if you buy and hold shares of the Fund. You may qualify for sales charge discounts if you invest at least $250,000 in Class T shares of the Fund in a single transaction. More information about these and other discounts is available from your financial professional and in the "Information on Sales Charges" section starting on page 111 of the Prospectus and in the "Alternative Sales Arrangements" section on page 108 of the Statement of Additional Information.

Shareholder Fees(fees paid directly from your investment)

Shareholder Fees	MainStay MacKay Emerging Markets Debt Fund Class T
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	2.50%
Maximum Deferred Sales Charge (Load) (as a percentage of the lesser of the original offering price or redemption proceeds)	none

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

Annual Fund Operating Expenses	MainStay MacKay Emerging Markets Debt Fund Class T
Management Fees (as an annual percentage of the Fund's average daily net assets)	0.73%	[1]
Distribution and/or Service (12b-1) Fees	0.25%
Other Expenses	0.24%
Total Annual Fund Operating Expenses	1.22%

Example

The Example is intended to help you compare the cost of investing in 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 whether or not you 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 that the Fund's operating expenses remain the same. The Example reflects the contractual fee waiver and/or expense reimbursement arrangement, if applicable, for the current duration of the arrangement only. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example	MainStay MacKay Emerging Markets Debt Fund Class T USD ($)
1 Year	$ 371
3 Years	628
5 Years	904
10 Years	$ 1,690

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 rate 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 37% of the average value of its portfolio.

Principal Investment Strategies

Under normal circumstances, the Fund invests at least 80% of its assets (net assets plus any borrowings for investment purposes) in fixed income securities of issuers in emerging markets. MacKay Shields LLC, the Fund's Subadvisor, has discretion to determine the countries considered to be emerging market countries, including taking into consideration a variety of factors such as the development of a country's financial and capital markets and inclusion in an index considered by the Subadvisor to be representative of emerging markets.

Some of the foreign securities in which the Fund invests may be denominated in foreign currency. The debt securities in which the Fund invests may consist of securities that are rated below investment grade. Below investment grade securities are generally securities that receive low ratings from an independent rating agency, such as rated lower than BBB- by Standard & Poor's Ratings Services and Baa3 by Moody's Investors Service, Inc., or if unrated, are deemed to be of comparable quality by the Fund's Subadvisor, MacKay Shields LLC. Securities rated below investment grade by independent rating agencies are commonly referred to as “high yield securities” or “junk bonds.” If independent rating agencies assign different ratings to the same security, the Fund will use the lower rating for purposes of determining the security's credit quality.

The Fund's principal investments include Yankee (dollar-denominated) debt securities, Brady Bonds, variable rate notes, mortgage-related and asset-backed securities and mortgage dollar rolls. The Fund may also invest in derivative instruments, such as floaters, including inverse floaters, forward commitments, futures, options and swap agreements to try to enhance returns or reduce the risk of loss by hedging certain of its holdings.

The Fund may invest up to 15% of its total assets in swaps, including credit default swaps. The Fund may buy and sell currency on a spot basis, buy foreign currency options, and enter into foreign currency forward contracts. These techniques may be used for any legally permissible purpose, including to increase the Fund's return.

In unusual market conditions, the Fund may invest all or a portion of its assets in fixed income securities of countries with established economies.

Investment Process: The Subadvisor identifies investment opportunities by beginning with country selection, then assessing local currencies for upside potential and downside risk and, finally, evaluating specific securities based on the financial condition and competitiveness of the issuer. The Subadvisor considers factors it deems relevant such as prospects for a country's political stability, currency exchange rates, interest rates, inflation, relative economic growth and/or governmental policies.

The Subadvisor may sell a security if it no longer believes the security will contribute to meeting the investment objective of the Fund. In considering whether to sell a security, the Subadvisor may evaluate, among other things, the condition of foreign economies and meaningful changes in the issuer's financial condition and competitiveness.

Principal Risks

You can lose money by investing in the Fund. An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency. The investments selected by the Fund's Subadvisor may underperform the market or other investments. The Fund may receive large purchase or redemption orders which may have adverse effects on performance if the Fund were required to sell securities, invest cash or hold a relatively large amount of cash at times when it would not otherwise do so.

The principal risks of investing in the Fund are summarized below.

Market Risk: The value of the Fund's investments may fluctuate because of changes in the markets in which the Fund invests, which could cause the Fund to underperform other funds with similar investment objectives and strategies. Changes in these markets may be rapid and unpredictable. From time to time, markets may experience periods of stress for potentially prolonged periods that may result in: (i) increased market volatility; (ii) reduced market liquidity; and (iii) increased redemptions of Fund shares. Such conditions may add significantly to the risk of volatility in the net asset value of the Fund's shares.

Portfolio Management Risk: The investment strategies, practices and risk analysis used by the Subadvisor may not produce the desired results. In addition, the Fund may not achieve its investment objective, including during a period in which the Subadvisor takes temporary positions in response to unusual or adverse market, economic or political conditions, or other unusual or abnormal circumstances.

Debt Securities Risk: The risks of investing in debt or fixed-income securities include (without limitation): (i) credit risk, e.g., the issuer or guarantor of a debt security may be unable or unwilling (or be perceived as unable or unwilling) to make timely principal and/or interest payments or otherwise honor its obligations; (ii) maturity risk, e.g., a debt security with a longer maturity may fluctuate in value more than one with a shorter maturity; (iii) market risk, e.g., low demand for debt securities may negatively impact their price; (iv) interest rate risk, e.g., when interest rates go up, the value of a debt security generally goes down, and when interest rates go down, the value of a debt security generally goes up (long-term debt securities are generally more susceptible to interest rate risk than short-term debt securities); and (v) call risk, e.g., during a period of falling interest rates, the issuer may redeem a security by repaying it early, which may reduce the Fund's income if the proceeds are reinvested at lower interest rates.

Interest rates in the United States are near historic lows, and the Fund currently faces a heightened level of interest rate risk. To the extent the Federal Reserve Board continues to raise the federal funds rate, there is a risk that interest rates across the financial system may rise, possibly significantly and/or rapidly. Rising interest rates or lack of market participants may lead to decreased liquidity and increased volatility in the fixed-income or debt markets, making it more difficult for the Fund to sell its fixed-income or debt holdings at a time when the Subadvisor might wish to sell. Decreased liquidity in the fixed-income or debt markets also may make it more difficult to value some or all of the Fund's fixed-income or debt holdings.

Not all U.S. government debt securities are guaranteed by the U.S. government-some are backed only by the issuing agency, which must rely on its own resources to repay the debt. The Fund's yield will fluctuate with changes in short-term interest rates.

Foreign Securities Risk: Investments in foreign securities may be riskier than investments in U.S. securities. Differences between U.S. and foreign regulatory regimes and securities markets, including less stringent investor protections and disclosure standards of some foreign markets, less liquid trading markets and political and economic developments in foreign countries, may affect the value of the Fund's investments in foreign securities. Foreign securities may also subject the Fund's investments to changes in currency rates. Changes in the value of foreign currencies may make the return on an investment go up or down, unrelated to the quality or performance of the investment itself.

Emerging Markets Risk: The risks related to investing in foreign securities are generally greater with respect to securities of companies that conduct their business activities in emerging markets or whose securities are traded principally in emerging markets. The risks of investing in emerging markets include the risks of illiquidity, increased price volatility, smaller market capitalizations, less government regulation, less extensive and less frequent accounting, financial and other reporting requirements, risk of loss resulting from problems in share registration and custody, substantial economic and political disruptions and the nationalization of foreign deposits or assets.

High-Yield Securities Risk: Investments in high-yield securities or non-investment grade securities (commonly referred to as "junk bonds") are considered speculative because they present a greater risk of loss than higher quality securities. Such securities may, under certain circumstances, be less liquid than higher rated securities. These securities pay investors a premium (a high interest rate or yield) because of the potential illiquidity and increased risk of loss. These securities can also be subject to greater price volatility. In times of unusual or adverse market, economic or political conditions, these securities may experience higher than normal default rates.

Mortgage-Related and Other Asset-Backed Securities Risk: Investments in mortgage-related securities (such as mortgage-backed securities) and other asset-backed securities generally involve a stream of payments based on the underlying obligations. These payments, which are often part interest and part return of principal, vary based on the rate at which the underlying borrowers repay their loans or other obligations. Asset-backed securities are subject to the risk that borrowers may default on the underlying obligations and that, during periods of falling interest rates, these obligations may be called or prepaid and, during periods of rising interest rates, obligations may be paid more slowly than expected. Impairment of the underlying obligations or collateral, such as by non-payment, will reduce the security's value. Enforcing rights against such collateral in events of default may be difficult or insufficient. The value of these securities may be significantly affected by changes in interest rates, the market's perception of issuers, and the creditworthiness of the parties involved. The ability of the Fund to successfully utilize these instruments may depend on the ability of the Subadvisor to forecast interest rates and other economic factors correctly. These securities may have a structure that makes their reaction to interest rate changes and other factors difficult to predict, making their value highly volatile.

Mortgage Dollar Roll Transaction Risk: A mortgage dollar roll is a transaction in which the Fund sells mortgage-related securities from its portfolio to a counterparty from whom it simultaneously agrees to buy a similar security on a delayed delivery basis. Mortgage dollar roll transactions are subject to certain risks, including the risk that securities returned to the Fund at the end of the roll, while substantially similar, may be inferior to what was initially sold to the counterparty.

Floaters and Variable Rate Notes Risk: Floaters and variable rate notes provide for a periodic adjustment in the interest rate paid on the securities. The rate adjustment intervals may be regular and range from daily up to annually, or may be based on an event, such as a change in the prime rate. Floating and variable rate notes may be subject to greater liquidity risk than other debt securities, meaning that there may be limitations on the Fund's ability to sell the securities at any given time. Securities with floating interest rates generally are less sensitive to interest rate changes, but may decline in value if their interest rates do not rise as much or as fast as interest rates in general. Such securities also may lose value.

Derivatives Risk: Derivatives are investments whose value depends on (or is derived from) the value of an underlying instrument, such as a security, asset, reference rate or index. Derivative strategies may expose the Fund to greater risk than if it had invested directly in the underlying instrument and often involve leverage, which may exaggerate a loss, potentially causing the Fund to lose more money than it originally invested and would have lost had it invested directly in the underlying instrument. Derivatives may be difficult to sell, unwind or value. Derivatives may also be subject to counterparty risk, which is the risk that the counterparty (the party on the other side of the transaction) on a derivative transaction will be unable to honor its contractual obligations to the Fund. Futures may be more volatile than direct investments in the instrument underlying the contract, and may not correlate perfectly to the underlying instrument. Futures and other derivatives also may involve a small initial investment relative to the risk assumed, which could result in losses greater than if they had not been used. Due to fluctuations in the price of the underlying asset, the Fund may not be able to profitably exercise an option and may lose its entire investment in an option. Forward commitments entail the risk that the instrument may be worth less when it is issued or received than the price the Fund agreed to pay when it made the commitment. The use of foreign currency forwards may result in currency exchange losses due to fluctuations in currency exchange rates or an imperfect correlation between portfolio holdings denominated in a particular currency and the forward contracts entered into by the Fund. Uncleared swaps are particularly subject to counterparty credit, correlation, valuation, liquidity and leveraging risks. Additionally, applicable regulators have adopted rules imposing certain margin requirements, including minimums on uncleared swaps, which may result in the Fund and its counterparties posting higher margin amounts for uncleared swaps. Certain standardized swaps are subject to mandatory central clearing and exchange trading. Central clearing, which interposes a central clearinghouse to each participant's swap, and exchange trading are intended to reduce counterparty credit risk and increase liquidity but neither makes swap transactions risk-free. Derivatives may also increase the expenses of the Fund.

Liquidity and Valuation Risk: Securities purchased by the Fund may be illiquid at the time of purchase or liquid at the time of purchase and subsequently become illiquid due to, among other things, events relating to the issuer of the securities, market events, operational issues, economic conditions, investor perceptions or lack of market participants. The lack of an active trading market may make it difficult to sell or obtain an accurate price for a security. If market conditions or issuer specific developments make it difficult to value securities, the Fund may value these securities using more subjective methods, such as fair value pricing. In such cases, the value determined for a security could be different than the value realized upon such security's sale. As a result, an investor could pay more than the market value when buying Fund shares or receive less than the market value when selling Fund shares. This could affect the proceeds of any redemption or the number of shares an investor receives upon purchase. Liquidity risk may also refer to the risk that the Fund may not be able to pay redemption proceeds within the allowable time period because of unusual market conditions, unusually high volume of redemptions, or other reasons. To meet redemption requests or to raise cash to pursue other investment opportunities, the Fund may be forced to sell securities at an unfavorable time and/or under unfavorable conditions, which may adversely affect the Fund.

Currency Risk: Changes in the value of foreign (non-U.S.) currencies relative to the U.S. dollar may adversely affect the Fund's investments in foreign currencies or in securities that trade in, and receive revenues in, or in derivatives that provide exposure to, foreign currencies. These changes in value can make the return on an investment go up or down, entirely apart from the quality or performance of the investment itself. One or more Subadvisors seek to reduce currency risk by hedging all or part of the exposure to various foreign currencies of the Fund's assets allocated to the Subadvisor(s) by engaging in hedging transactions, including swaps, futures, forward currency contracts and other derivatives. One or more Subadvisors may from time to time attempt to hedge all or a portion of the perceived currency risk by engaging in similar hedging transactions. However, these transactions and techniques may not always work as intended, and in certain cases the Fund may be worse off than if it had not engaged in such hedging practices. In addition, certain market conditions may make it impossible or uneconomical to hedge against currency risk.

Money Market/Short-Term Securities Risk: To the extent the Fund holds cash or invests in money market or short-term securities, the Fund may be less likely to achieve its investment objective. In addition, it is possible that the Fund's investments in these instruments could lose money.

Past Performance

Class T shares have not commenced operations as of the date of this Prospectus. As a result, the following chart and table give you a picture of the performance for the other share classes of the Fund, which are not offered in this Prospectus. The performance of Class T shares would be similar to the other share classes because all share classes are invested in the same portfolio of securities and would differ only to the extent that Class T shares have different expenses. To the extent Class T shares have higher expenses than the other share classes of the Fund, its performance would be lower.

The following bar chart and table indicate some of the risks of investing in the Fund. The bar chart shows you how the Fund's calendar year performance has varied over the last ten years. Sales loads, if any, are not reflected in the bar chart. If they were, returns would be less than those shown. The average annual total returns table shows how the Fund's average annual total returns (before and after taxes) for the one-, five- and ten-year periods compare to those of a broad-based securities market index. The Fund has selected the JPMorgan EMBI Global Diversified Indexas its primary benchmark. The JPMorgan EMBI Global Diversified Index is a market capitalization weighted, total return index tracking the traded market for U.S. dollar-denominated Brady Bonds, Eurobonds, traded loans and local market debt instruments issued by sovereign and quasi-sovereign entities.

Performance data for the classes varies based on differences in their fee and expense structures. Performance is not shown for classes with less than one calendar year of performance. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. Please visit nylinvestments.com/funds for more recent performance information.

Effective February 28, 2017, the Fund revised its principal investment strategies. The performance in the bar chart and table prior to that date reflects the Fund's prior principal investment strategies. Past performance may have been different if the revised principal investment strategies had been in place prior to that date.

Annual Returns, Class B Shares

(by calendar year 2008-2017)

Best Quarter
3Q/09	13.07	%
Worst Quarter
4Q/08	-8.94	%

Average Annual Total Returns (for the periods ended December 31, 2017)

Average Annual Total Returns - - MainStay MacKay Emerging Markets Debt Fund	1 Year	5 Years	10 Years	Inception Date
Class B	5.26%	1.71%	5.40%	Jun. 01, 1998
Class B | After Taxes on Distributions	4.14%	(0.17%)	3.34%
Class B | After Taxes on Distributions and Sales	2.96%	0.43%	3.36%
Class A	6.32%	2.05%	5.88%	Jun. 01, 1998
Investor Class	6.10%	1.86%	5.81%	Feb. 28, 2008
Class C	9.14%	2.02%	5.41%	Sep. 01, 1998
Class I	11.59%	3.25%	6.62%	Aug. 31, 2007
JPMorgan EMBI Global Diversified Index (reflects no deductions for fees, expenses, or taxes)	10.26%	4.58%	7.29%

After-tax returns are calculated using the highest individual federal marginal income tax rates in effect at the time of each distribution or capital gain or upon the sale of fund shares, and do not reflect the impact of state and local taxes. In some cases, the return after taxes may exceed the return before taxes due to an assumed tax benefit from any losses on a sale of shares at the end of the measurement period. Actual after-tax returns depend on your tax situation and may differ from those shown. After-tax returns are not relevant if you hold your shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. After-tax returns shown are for Class B shares. After-tax returns for the other share classes may vary.

[1] The management fee is as follows: 0.70% on assets up to $500 million and 0.65% on assets over $500 million, plus a fee for fund accounting services previously provided by New York Life Investment Management LLC under a separate fund accounting agreement. This addition to the management fee amounted to 0.02% of the Fund's average daily net assets, but did not result in a net increase in Total Annual Fund Operating Expenses.