The article discusses the concept of market efficiency, which suggests that stock prices reflect all available information. The authors explore different versions of this hypothesis and how it relates to the costs of obtaining and acting on information. They argue that while the extreme version of market efficiency may not hold true due to information and trading costs, it still serves as a useful benchmark for understanding price adjustments. The main challenge in testing market efficiency lies in the need to consider it alongside an asset-pricing model, as anomalies in returns could be attributed to either market inefficiency or a flawed model of market equilibrium.