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The efficient market hypothesis (EMH), alternatively known as the efficient market theory, is a hypothesis that states that share prices reflect all information and consistent alpha generation is impossible.
According to the EMH, stocks always trade at their fair value on exchanges, making it impossible for investors to purchase undervalued stocks or sell stocks for inflated prices. Therefore, it should be impossible to outperform the overall market through expert stock selection or market timing, and the only way an investor can obtain higher returns is by purchasing riskier investments.
Although it is a cornerstone of modern financial theory, the EMH is highly controversial and often disputed. Believers argue it is pointless to search for undervalued stocks or to try to predict trends in the market through either fundamental or technical analysis.
Theoretically, neither technical nor fundamental analysis can produce risk-adjusted excess returns (alpha) consistently, and only inside information can result in outsized risk-adjusted returns.
The June 27, 2022 share price of the most expensive stock in the world: Berkshire Hathaway Inc. Class A (BRK.A).
While academics point to a large body of evidence in support of EMH, an equal amount of dissension also exists. For example, investors such as Warren Buffett have consistently beaten the market over long periods, which by definition is impossible according to the EMH.
Detractors of the EMH also point to events such as the 1987 stock market crash, when the Dow Jones Industrial Average (DJIA) fell by over 20% in a single day, and asset bubbles as evidence that stock prices can seriously deviate from their fair values.
The assumption that markets are efficient is a cornerstone of modern financial economics—one that has come under question in practice.
Proponents of the Efficient Market Hypothesis conclude that, because of the randomness of the market, investors could do better by investing in a low-cost, passive portfolio.
Data compiled by Morningstar Inc., in its June 2019 Active/Passive Barometer study, supports the EMH. Morningstar compared active managers’ returns in all categories against a composite made of related index funds and exchange-traded funds (ETFs). The study found that over a 10 year period beginning June 2009, only 23% of active managers were able to outperform their passive peers. Better success rates were found in foreign equity funds and bond funds. Lower success rates were found in US large-cap funds. In general, investors have fared better by investing in low-cost index funds or ETFs.
While a percentage of active managers do outperform passive funds at some point, the challenge for investors is being able to identify which ones will do so over the long term. Less than 25 percent of the top-performing active managers can consistently outperform their passive manager counterparts over time.
Market efficiency refers to how well prices reflect all available information. The efficient markets hypothesis (EMH) argues that markets are efficient, leaving no room to make excess profits by investing since everything is already fairly and accurately priced. This implies that there is little hope of beating the market, although you can match market returns through passive index investing.
The validity of the EMH has been questioned on both theoretical and empirical grounds. There are investors who have beaten the market, such as Warren Buffett, whose investment strategy focused on undervalued stocks made billions and set an example for numerous followers. There are portfolio managers who have better track records than others, and there are investment houses with more renowned research analysis than others. EMH proponents, however, argue that those who outperform the market do so not out of skill but out of luck, due to the laws of probability: at any given time in a market with a large number of actors, some will outperform the mean, while others will underperform.
There are certainly some markets that are less efficient than others. An inefficient market is one in which an asset's prices do not accurately reflect its true value, which may occur for several reasons. Market inefficiencies may exist due to information asymmetries, a lack of buyers and sellers (i.e. low liquidity), high transaction costs or delays, market psychology, and human emotion, among other reasons. Inefficiencies often lead to deadweight losses. In reality, most markets do display some level of inefficiencies, and in the extreme case an inefficient market can be an example of a market failure.
Accepting the EMH in its purest (strong) form may be difficult as it states that all information in a market, whether public or private, is accounted for in a stock's price. However, modifications of EMH exist to reflect the degree to which it can be applied to markets:

The more participants are engaged in a market, the more efficient it will become as more people compete and bring more and different types of information to bear on the price. As markets become more active and liquid, arbitrageurs will also emerge, profiting by correcting small inefficiencies whenever they might arise and quickly restoring efficiency.
The Library of Economics and Liberty. "Efficient Capital Markets."
Yahoo Finance. "Berkshire Hathaway Inc. (BRK-A)."
Federal Reserve History. "Stock Market Crash of 1987."
Morningstar. "Active Funds vs. Passive Funds: Which Fund Types Had Increased Success Rates?"
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