Using a nationally representative longitudinal survey of lawyers in the U.S., we document a sizeable gap between men and women in their early aspirations to become law firm partners, despite similar early investments and educational characteristics. This aspiration gap can explain a large part of the gender promotion gap that is observed later. We propose a model to understand the role of aspirations and then empirically test its predictions. We show that aspirations create incentives to exert effort and are correlated with expectations of success and the preference for becoming a partner. We further show that aspirations are affected by early work experiences - facing harassment or demeaning comments early in the career affects long-term promotion outcomes mediated via aspirations. Our research highlights the importance of accounting for, and managing, career aspirations as an early intervention to close gender career gaps.