Researchers have long argued over whether strategies based on past stock returns have power to explain future stock returns. This paper finds no convincing evidence that either short-run or long-run contrarian strategies represent important factors for explaining the cross-section of stock returns. In contrast, the properly specified one-year momentum strategy has explanatory power for stock returns when used alone, when tested against size and book-to-market, and when subjected to exhaustive robustness checks. We conclude that one-year momentum represents a necessary third factor, along with firm size and book-to-market, for explaining the cross-section of stock returns.