The document discusses the efficient market hypothesis (EMH) which argues that stock prices reflect all available information. It defines three forms of market efficiency - weak, semi-strong, and strong - based on the types of information reflected in stock prices. The weak form states that prices reflect all historical price data, while the semi-strong form argues that prices immediately incorporate publicly available information. Empirical tests provide mixed support for the different forms of the EMH. The document also discusses potential market inefficiencies and anomalies that appear to contradict the EMH, such as the size effect and January effect.