A critical void in the research on women's underrepresentation in elective office is an analysis of the initial decision to run for office. Based on data from our Citizen Political Ambition Study, the first large-scale national survey of potential candidates, we examine the process by which women and men emerge as candidates for public office. We find that women who share the same personal characteristics and professional credentials as men express significantly lower levels of political ambition to hold elective office. Two factors explain this gender gap: first, women are far less likely than men to be encouraged to run for office; second, women are significantly less likely than men to view themselves as qualified to run. Our findings call into question the leading theoretical explanations for women's numeric underrepresentation and indicate that, because of vestiges of traditional sex-role socialization, prospects for gender parity in U.S. political institutions are less promising than conventional explanations suggest.