In this paper we examine the circumstances and determinants of female migration between Mexico and the United States. Using data from the Mexican Migration Project, we considered the relative timing of males' and females' moves northward. We then estimated logit and probit models to study the determinants of male and female out-migration; among women we also estimated a multinomial logit model to uncover differences in the process of migration for work versus not for work. We found that women almost always followed other family members, either the husband or a parent; only a tiny minority initiated migration independently. Although males also are quite likely to be introduced to migration by a parent, nearly half of all male migrants left for the United States before or without a wife or a parent. Estimates of the determinants of migration suggested that males move for employment, whereas wives generally are motivated by family reasons. Daughters, however, display a greater propensity to move for work, and the determinants of their work-related moves closely resemble those of sons and fathers.