Using rich data on personnel records, work assignments, and performance in a finan- cial institution, we uncover the mechanisms leading to promotion gaps in knowledge teamwork. We find a substantial promotion gap for women in early career stages. Analyzing over 10,000 investment projects reveals that assignments to project team leaderships (a “promotable” task) are crucial in explaining the gaps in promotions and affect long-term careers. We find causal evidence that male supervisors favor male bankers, while women benefit from female supervisors. A survey among employees indicates that women perceive to be disadvantaged in the assignments of tasks, but they do not differ in aspirations and demand for these roles. JEL codes: M51, J16, D22, J44 Keywords: Careers; gender gaps; visibility; leadership; internal labor market (work joint with Cagatay Bircan and Tristan Stahl)