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Weak-form efficiency: past prices don't predict future returns (rules out technical analysis). Semi-strong: public information is instantly priced (rules out fundamental analysis for excess returns). Strong-form: all information (public and private) is priced (rare empirically). Most evidence supports semi-strong efficiency.
From the Efficient Market Hypothesis, you know the core claim: in competitive markets, prices quickly incorporate available information, making it impossible to consistently earn abnormal returns. The three forms of market efficiency — weak, semi-strong, and strong — define three progressively larger information sets, and they are nested: semi-strong efficiency implies weak-form efficiency, and strong-form implies both.
Weak-form efficiency says that current prices already reflect all information contained in the history of past prices and trading volume. If this holds, looking at a chart of past prices — the entire enterprise of technical analysis — cannot generate systematically better-than-market returns. The price tomorrow is not predictable from the price today because all the predictive content of yesterday's price was already priced in by the market. This form is empirically well-supported: price series in developed markets look close to a random walk, and trading rules based on chart patterns (head-and-shoulders, moving average crossovers) generally fail to beat a buy-and-hold strategy after transaction costs.
Semi-strong efficiency raises the bar: prices reflect all *publicly available* information, not just price history. This includes earnings announcements, news, analyst reports, economic data — anything anyone can read. If the market is semi-strong efficient, fundamental analysis (studying financial statements, forecasting cash flows, and estimating intrinsic value) cannot reliably generate excess returns, because by the time you act on public information, the market has already incorporated it. The evidence here is strong but less decisive: event studies show that stock prices react almost instantaneously to earnings surprises and news releases, leaving little room for profit. However, documented anomalies — the value premium, the momentum effect — have been interpreted by some researchers as evidence against semi-strong efficiency and by others as risk factors not captured by standard models.
Strong-form efficiency claims that prices reflect all information, including private (inside) information. This is the most extreme version and is clearly violated: insider trading laws exist precisely because traders with material non-public information can profit from it — and enforcement actions confirm that they do. Empirically, strong-form efficiency is rejected. The practical hierarchy, then, is: weak-form efficiency is the baseline that most evidence supports; semi-strong efficiency is the working assumption of well-functioning public markets with some empirical anomalies at the edges; strong-form efficiency is a theoretical extreme that does not hold. The key policy implication is that insider trading regulations are necessary precisely because strong-form efficiency does not automatically police itself.