We examine the process of stock prices adjusting to information conveyed by the trading process. Using the price impact of a trade to measure its information content, our analysis shows that the weekly price impact of market transactions has significant cross-sectional predictive power for returns in the subsequent week. The effect is sensitive to the level of informational asymmetry and is not due to excess liquidity demands or variations in rational risk premia. This finding suggests that prices may slowly incorporate trading information. We then characterize the key channel through which price underreaction occurs. We find that the price impact contains information that is not fully captured by public order flows and that a lead-lag effect exists regarding the arrival of information to different groups of investors. Hong and Stein’s (1999) gradual-information-diffusion theory seems the most likely explanation for price underreaction.