This paper reviews some of the past literature on male-female wage differentials in order to determine the early hypotheses which are the historical roots of the current theoretical and empirical work analyzing male-female wage differentials. Part 1 reviews the discrimination hypotheses, which emphasize differences in the labor market conditions (such as crowding, job segmentation, and lack of organization) that result in different rates of pay for the same work. Part 2 reviews the human capital hypotheses, which assume that the sexes have different productive attributes that yield wage differentials. This discussion focuses on socialization, labor force commitment, investment in human capital, differential female supply prices, and physical strength differences. Part 3 provides a summary and conclusion suggesting that policy directed toward reducing male-female wage differentials should be broad in scope and address itself to the numerous causes of the differentials indicated in parts 1 and 2. (BM)