Recently, there has been much empirical interest in why women are so poorly represented in executive ranks in organizations. This review summarizes the findings from recent empirical studies into the causes of gender differences in advancement to executive levels. The results are categorized to answer three questions based on the major arguments for, and assumptions made about, gender differences in advancement to the top of organizations. Are there gender differences in advancing to the top of organizations because (1) women lack the relevant knowledge, skills and expertise, that is, human capital, (2) women lack the relevant networks, are stereotyped as unsuitable, and are in male organizational cultures, that is, social capital, and/or (3) different factors are needed to advance to higher than to lower levels; women incrementally develop fewer of these factors and resources than men, and thus advance less to the top? The review most supports the second proposition. Surprisingly, even though there are many studies, several critical questions have not been addressed with strong research designs. Research using rigorous designs is especially needed to test the major theoretical frameworks.