For years, investment professionals have used stock factors, such as price-to-book ratio, to help make stock selection decisions. If a stock factor is associated with ex post risk-adjusted returns, it is said to be anomalous, and anomalous stock factors can often be profitably exploited. In this monograph, the author investigates factor-return relationships by looking at the rationales for their existence and by drawing on a database of global equity market data. The results of this analysis will comfort some readers while surprising others.