Pioneer Mid Cap Value Fund

Class A Shares (PCGRX)	Class C Shares (PCCGX)	Class K Shares (PMCKX)
Class R Shares (PCMRX)	Class T Shares (——-)	Class Y Shares (PYCGX)

Summary Prospectus

March 1, 2018

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 http://us.pioneerinvestments.com/misc/prospectus.jsp. You also can obtain this information at no cost by calling 1-800-225-6292 or by sending an email request to ask.amundipioneer@amundipioneer.com. The fund’s current prospectus and statement of additional information, dated March 1, 2018, and the independent registered public accounting firm’s report and financial statements in the fund’s annual report to shareholders dated October 31, 2017, are incorporated by reference into this summary prospectus.

Investment objective

Capital appreciation by investing in a diversified portfolio of securities consisting primarily of common stocks.

Fees and expenses of the fund

This table 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 or your family invest, or agree to invest in the future, at least $50,000 in Class A shares of the Pioneer funds. More information about these and other discounts is available from your investment professional and in the “Sales charges” section of the prospectus beginning on page 40, the “Intermediary defined sales charge waiver policies” section of the prospectus beginning on page 80, and the “Sales charges” section of the statement of additional information beginning on page 55.

Summary Prospectus

Shareowner fees
(fees paid directly from your investment)	Class A	Class C	Class K	Class R	Class T	Class Y
Maximum sales charge (load) when you buy shares (as a percentage of offering price)	5.75%	None	None	None	2.50%	None
Maximum deferred sales charge (load) (as a percentage of offering price or the amount you receive when you sell shares, whichever is less)	None¹	1%	None	None	None	None

Annual fund operating expenses
(expenses that you pay each year as a percentage of the value of your investment)	Class A	Class C	Class K	Class R	Class T	Class Y
Management Fees	0.60%	0.60%	0.60%	0.60%	0.60%	0.60%
Distribution and Service (12b-1) Fees	0.25%	1.00%	0.00%	0.50%	0.25%	0.00%
Other Expenses²	0.19%	0.27%	0.08%	0.36%	0.19%	0.21%
Total Annual Fund Operating Expenses	1.04%	1.87%	0.68%	1.46%	1.04%	0.81%

1	Class A purchases of $500,000 or more that are not subject to an initial sales charge may be subject to a contingent deferred sales charge of 1%. See “Sales charges.”
2	Other Expenses for Class T shares are based on estimated amounts for the current fiscal year.

Example

This 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 shown and then, except as indicated, redeem all of your shares at the end of those periods. It also assumes that (a) your investment has a 5% return each year and (b) the fund's total annual operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

	If you redeem your shares				If you do not redeem your shares
	Number of years you own your shares
	1	3	5	10	1	3	5	10
Class A	$675	$887	$1,116	$1,773	$675	$887	$1,116	$1,773
Class C	290	588	1,011	2,190	190	588	1,011	2,190
Class K	69	218	379	847	69	218	379	847
Class R	149	462	797	1,746	149	462	797	1,746
Class T	353	573	810	1,489	353	573	810	1,489
Class Y	83	259	450	1,002	83	259	450	1,002

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 58% of the average value of its portfolio.

Principal investment strategies

Normally, the fund invests at least 80% of its total assets in equity securities of mid-size companies. Mid-size companies are those with market values, at the time of investment, that do not exceed the greater of the market capitalization of the largest company within the Russell Midcap Value Index ($36.68 billion as of December 31, 2017) or the 3-year rolling average of the market capitalization of the largest company within the Russell Midcap Value Index ($33.38 billion as of December 31, 2017), as measured at the end of the preceding month, and are not less than the smallest company within the index. The Russell Midcap Value Index measures the performance of U.S. mid-cap value stocks. The size of the

Summary Prospectus

companies in the index changes constantly with market conditions and the composition of the index. The equity securities in which the fund principally invests are common stocks, preferred stocks and depositary receipts, but the fund may invest in other types of equity securities to a lesser extent, such as funds that invest primarily in equity securities, equity interests in real estate investment trusts (REITs), warrants and rights. The fund may invest in initial public offerings of equity securities.

The fund may invest up to 25% of its total assets in securities of non-U.S. issuers. The fund will not invest more than 5% of its total assets in the securities of emerging markets issuers.

The fund may invest up to 20% of its net assets in REITs.

The fund may invest up to 20% of its total assets in debt securities. The fund may invest up to 5% of its net assets in below investment grade debt securities (known as “junk bonds”), including below investment grade convertible debt securities.

The fund may, but is not required to, use derivatives, such as stock index futures and options. The fund may use derivatives for a variety of purposes, including; in an attempt to hedge against adverse changes in the market price of securities, interest rates or currency exchange rates; as a substitute for purchasing or selling securities; to attempt to increase the fund's return as a non-hedging strategy that may be considered speculative; to manage portfolio characteristics; and as a cash flow management technique. The fund may choose not to make use of derivatives for a variety of reasons, and any use may be limited by applicable law and regulations. The fund may also hold cash or other short-term investments.

The fund uses a “value” style of management. The adviser seeks to identify securities that are selling at reasonable prices or at substantial discounts to their underlying values and then holds these securities until the market values reflect their intrinsic values. The adviser evaluates a security's potential value, including the attractiveness of its market valuation, based on the company's assets and prospects for earnings growth. In making that assessment, the adviser employs fundamental research and an evaluation of the issuer based on its financial statements and operations, employing a bottom-up analytic style, which focuses on specific securities rather than

on industries. The adviser focuses on the quality and price of individual issuers and securities. The adviser generally sells a portfolio security when it believes that the security’s market value reflects its underlying value.

Principal risks of investing in the fund

You could lose money on your investment in the fund. As with any mutual fund, there is no guarantee that the fund will achieve its objective.

Market risk. The value of securities held by the fund may go up or down, sometimes rapidly or unpredictably, due to general market conditions, such as real or perceived adverse economic, political, or regulatory conditions, inflation, changes in interest or currency rates, lack of liquidity in the bond markets or adverse investor sentiment. In the past decade, financial markets throughout the world have experienced increased volatility, depressed valuations, decreased liquidity and heightened uncertainty. Governmental and non-governmental issuers have defaulted on, or been forced to restructure, their debts. These conditions may continue, recur, worsen or spread. Events that have contributed to these market conditions include, but are not limited to, major cybersecurity events; geopolitical events (including wars and terror attacks); measures to address budget deficits; downgrading of sovereign debt; declines in oil and commodity prices; dramatic changes in currency exchange rates; and public sentiment. The U.S. government and the Federal Reserve, as well as certain foreign governments and their central banks, have taken steps to support financial markets, including by keeping interest rates at historically low levels. This and other government intervention may not work as intended, particularly if the efforts are perceived by investors as being unlikely to achieve the desired results. The Federal Reserve has reduced its market support activities and has begun raising interest rates. Certain foreign governments and central banks are implementing or discussing so-called negative interest rates (e.g., charging depositors who keep their cash at a bank) to spur economic growth. Further Federal Reserve or other U.S. or non-U.S. governmental or central bank actions, including interest rate increases or contrary actions by different governments, could negatively affect financial markets generally, increase market volatility and reduce the value and liquidity of securities in which the fund invests. Policy and legislative changes in the U.S. and in other countries are affecting many aspects of financial regulation, and may in some instances contribute to decreased liquidity and increased volatility in the financial markets. The impact of these changes on the markets, and the practical implications for market

Summary Prospectus

participants, may not be fully known for some time. Economies and financial markets throughout the world are increasingly interconnected. Economic, financial or political events, trading and tariff arrangements, terrorism, natural disasters and other circumstances in one country or region could have profound impacts on global economies or markets. As a result, whether or not the fund invests in securities of issuers located in or with significant exposure to the countries directly affected, the value and liquidity of the fund’s investments may be negatively affected. The fund may experience a substantial or complete loss on any individual security or derivative position.

Mid-size companies risk. Compared to large companies, mid-size companies, and the market for their equity securities, may be more sensitive to changes in earnings results and investor expectations, have more limited product lines and capital resources, experience sharper swings in market values, be harder to sell at the times and prices the adviser thinks appropriate, and offer greater potential for gain and loss.

Value style risk. The prices of securities the adviser believes are undervalued may not appreciate as expected or may go down. Value stocks may fall out of favor with investors and underperform the overall equity market.

Portfolio selection risk. The adviser's judgment about a particular security or issuer, or about the economy or a particular sector, region or market segment, or about an investment strategy, may prove to be incorrect.

Risks of non-U.S. investments. Investing in non-U.S. issuers, or in U.S. issuers that have significant exposure to foreign markets, may involve unique risks compared to investing in securities of U.S. issuers. These risks are more pronounced for issuers in emerging markets or to the extent that the fund invests significantly in one region or country. These risks may include different financial reporting practices and regulatory standards, less liquid trading markets, extreme price volatility, currency risks, changes in economic, political, regulatory and social conditions, terrorism, sustained economic downturns, financial instability, tax burdens, and investment and repatriation restrictions. Lack of information and less market regulation also may affect the value of these securities. Withholding and other non-U.S. taxes may decrease the fund’s return. Non-U.S. issuers may be located in parts of the world that have historically been prone to natural disasters. Investing in depositary receipts is subject to many of the

same risks as investing directly in non-U.S. issuers. Depositary receipts may involve higher expenses and may trade at a discount (or premium) to the underlying security. A number of countries in the European Union (EU) have experienced, and may continue to experience, severe economic and financial difficulties. In addition, voters in the United Kingdom have approved withdrawal from the EU. Other countries may seek to withdraw from the EU and/or abandon the euro, the common currency of the EU.

Risks of investments in real estate related securities. Investments in real estate securities are affected by economic conditions, interest rates, governmental actions and other factors. In addition, investing in REITs involves unique risks. They are significantly affected by the market for real estate and are dependent upon management skills and cash flow. REITs may have lower trading volumes and may be subject to more abrupt or erratic price movements than the overall securities markets. Mortgage REITs are particularly subject to interest rate and credit risks. In addition to its own expenses, the fund will indirectly bear its proportionate share of any management and other expenses paid by REITs in which it invests. Many real estate companies, including REITs, utilize leverage.

Risks of initial public offerings. Companies involved in initial public offerings (IPOs) generally have limited operating histories, and prospects for future profitability are uncertain. The market for IPO issuers has been volatile, and share prices of newly public companies have fluctuated significantly over short periods of time. The purchase of IPO shares may involve high transaction costs.

Risks of investment in other funds. Investing in other investment companies, including exchange-traded funds (ETFs), subjects the fund to the risks of investing in the underlying securities or assets held by those funds. When investing in another fund, the fund will bear a pro rata portion of the underlying fund’s expenses, in addition to its own expenses.

Risks of convertible securities. The market values of convertible securities tend to decline as interest rates increase and, conversely, to increase as interest rates decline. A downturn in equity markets may cause the price of convertible securities to decrease relative to other fixed income securities.

Preferred stocks risk. Preferred stocks may pay fixed or adjustable rates of return. Preferred stocks are subject to issuer-specific and market risks applicable generally to equity securities. In addition, a company’s preferred

Summary Prospectus

stocks generally pay dividends only after the company makes required payments to holders of its bonds and other debt. Thus, the value of preferred stocks will usually react more strongly than bonds and other debt to actual or perceived changes in the company’s financial condition or prospects. The market value of preferred stocks generally decreases when interest rates rise. Preferred stocks of smaller companies may be more vulnerable to adverse developments than preferred stocks of larger companies.

Risks of warrants and rights. If the price of the underlying stock does not rise above the exercise price before the warrant expires, the warrant generally expires without any value and the fund loses any amount it paid for the warrant. The failure to exercise subscription rights to purchase common shares would result in the dilution of the fund’s interest in the issuing company.

Debt securities risk. Factors that could contribute to a decline in the market value of debt securities in the fund include rising interest rates, if the issuer or other obligor of a security held by the fund fails to pay principal and/or interest, otherwise defaults or has its credit rating downgraded or is perceived to be less creditworthy or the credit quality or value of any underlying assets declines. Interest rates in the U.S, recently have been historically low and are expected to continue to rise. Junk bonds have a higher risk of default or are already in default and are considered speculative.

Market segment risk. To the extent the fund emphasizes, from time to time, investments in a market segment, the fund will be subject to a greater degree to the risks particular to that segment, and may experience greater market fluctuation than a fund without the same focus.

Industries in the financial segment, such as banks, insurance companies, broker-dealers and real estate investment trusts (REITs), may be sensitive to changes in interest rates and general economic activity and are generally subject to extensive government regulation.

Derivatives risk. Using stock index futures and options and other derivatives can increase fund losses and reduce opportunities for gains when market prices, interest rates or the derivative instruments themselves behave in a way not anticipated by the fund. Using derivatives may increase the volatility of the fund's net asset value and may not provide the result intended. Derivatives may have a leveraging effect on

the fund. Some derivatives have the potential for unlimited loss, regardless of the size of the fund’s initial investment. Changes in a derivative’s value may not correlate well with the referenced asset or metric. The fund also may have to sell assets at inopportune times to satisfy its obligations. Derivatives may be difficult to sell, unwind or value, and the counterparty may default on its obligations to the fund. Use of derivatives may have different tax consequences for the fund than an investment in the underlying security, and such differences may affect the amount, timing and character of income distributed to shareholders. The U.S. government and foreign governments are in the process of adopting and implementing regulations governing derivatives markets, including mandatory clearing of certain derivatives, margin and reporting requirements. The ultimate impact of the regulations remains unclear. Additional regulation of derivatives may make them more costly, limit their availability or utility, otherwise adversely affect their performance or disrupt markets.

Leveraging risk. The value of your investment may be more volatile and other risks tend to be compounded if the fund borrows or uses derivatives or other investments, such as ETFs, that have embedded leverage. Leverage generally magnifies the effect of any increase or decrease in the value of the fund's underlying assets and creates a risk of loss of value on a larger pool of assets than the fund would otherwise have, potentially resulting in the loss of all assets. Engaging in such transactions may cause the fund to liquidate positions when it may not be advantageous to do so to satisfy its obligations or meet segregation requirements.

Portfolio turnover risk. If the fund does a lot of trading, it may incur additional operating expenses, which would reduce performance. A higher level of portfolio turnover may also cause shareholders to incur a higher level of taxable income or capital gains.

Valuation risk. The sales price the fund could receive for any particular portfolio investment may differ from the fund’s valuation of the investment, particularly for illiquid securities and securities that trade in thin or volatile markets or that are valued using a fair value methodology. Investors who purchase or redeem fund shares on days when the fund is holding fair-valued securities may receive fewer or more shares or lower or higher redemption proceeds than they would have received if the fund had not fair-valued the securities or had used a different valuation methodology.

Summary Prospectus

The fund’s ability to value its investments may also be impacted by technological issues and/or errors by pricing services or other third party service providers.

Liquidity risk. Some securities and derivatives held by the fund may be impossible or difficult to purchase, sell or unwind, particularly during times of market turmoil. Illiquid securities and derivatives also may be difficult to value. If the fund is forced to sell an illiquid asset or unwind a derivatives position to meet redemption requests or other cash needs, the fund may be forced to sell at a loss.

Redemption risk. The fund may experience heavy redemptions that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value, which could cause the value of your investment to decline.

Expense risk. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if overall net assets decrease. Net assets are more likely to decrease and fund expense ratios are more likely to increase when markets are volatile.

Please note that there are many other factors that could adversely affect your investment and that could prevent the fund from achieving its goals.

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 government agency.

The fund's past performance

The bar chart and table indicate the risks and volatility of an investment in the fund by showing how the fund has performed in the past. The bar chart shows changes in the performance of the fund's Class A shares from calendar year to calendar year. The table shows the average annual total returns for each class of the fund over time and compares these returns to the returns of the Russell Midcap Value Index, a broad-based measure of market performance that has characteristics relevant to the fund’s investment strategies. You can obtain updated performance information by visiting https://us.pioneerinvestments.com/performance or by calling 1-800-225-6292.

The fund's past performance (before and after taxes) does not necessarily indicate how it will perform in the future.

The fund began focusing on mid-cap securities during 1999.

The bar chart does not reflect any sales charge you may pay when you buy fund shares. If this amount was reflected, returns would be less than those shown.

Annual return Class A shares (%)

(Year ended December 31)

For the period covered by the bar chart:

The highest calendar quarterly return was 16.62% (07/01/2009 to 09/30/2009).
The lowest calendar quarterly return was –21.57% (10/01/2008 to 12/31/2008).

No performance information is presented for Class T shares in the table below because Class T shares do not have annual returns for at least one calendar year. The returns for Class T shares would differ from those of other classes of shares because they have different expenses.

Summary Prospectus

Average annual total return (%)

(for periods ended December 31, 2017)

	1 Year	5 Years	10 Years	Since Inception	Inception Date
Class A					7/25/90
Return before taxes	6.37	12.00	5.98	10.30
Return after taxes on distributions	4.31	9.56	4.72	8.54
Return after taxes on distributions and sale of shares	5.12	9.18	4.57	8.32
Class C†	11.89	12.38	5.69	7.70	1/31/96
Class K	13.23	N\A	N\A	7.17	3/2/15
Class R	12.35	12.89	6.25	9.81	4/1/03
Class Y	13.12	13.69	6.99	8.22	7/2/98
Russell Midcap Value Index (reflects no deduction for fees, expenses or taxes)	13.34	14.68	9.10	12.08	7/25/90

†	The performance of Class C shares does not reflect the 1% front-end sales charge in effect prior to February 1, 2004. If you paid a 1% sales charge, your returns would be lower than those shown above.

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. Actual after-tax returns depend on the investor's tax situation and may differ from those shown. The after-tax returns shown are not relevant to investors who hold fund shares through tax-deferred arrangements such as 401(k) plans or individual retirement accounts.

After-tax returns are shown only for Class A shares. After-tax returns for Class C, Class K, Class R, Class T and Class Y shares shares will vary.

Management

Investment adviser	Amundi Pioneer Asset Management, Inc.
Portfolio management	Edward T. Shadek Jr., Senior Vice President and Director of Mid Cap Value, U.S. of Amundi Pioneer (lead portfolio manager of the fund since 2013) and Raymond Haddad, Vice President of Amundi Pioneer (portfolio manager of the fund since March 2018)

Purchase and sale of fund shares

You may purchase, exchange or sell (redeem) shares each day the New York Stock Exchange is open through your financial intermediary or, for accounts held directly with the fund, by contacting the fund in writing or by telephone: Pioneer Funds, P.O. Box 55014, Boston, MA 02205-5014, tel. 1-800-225-6292.

Your initial investment for Class A, Class C or Class T shares must be at least $1,000. Additional investments must be at least $100 for Class A and Class T shares and $500 for Class C shares. The initial investment for Class K shares and Class Y shares must be at least $5 million. This amount may be invested in one or more of the Pioneer mutual funds that currently offer Class K shares or Class Y shares, as applicable. There is no minimum additional investment amount for Class K or Class Y shares. There is no minimum investment amount for Class R shares.

Tax information

The fund intends to make distributions that may be taxed as ordinary income, qualified dividend income, or capital gains.

Payments to broker-dealers and other financial intermediaries

If you purchase 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 fund shares and related services. These payments create a conflict of interest by influencing the broker-dealer or other intermediary and your salesperson or investment professional to recommend the fund over another investment. Ask your salesperson or investment professional or visit your financial intermediary’s website for more information.

Notes

23458-08-0318