This chapter contributes to a better understanding of the gender wage gap over women’s professional career by focusing on the gap in pay between similarly-skilled women and men within and between firms at each age. Using linked employer-employee data for sixteen OECD countries, it is shown that about three quarters of the gender wage gap reflects pay differences within firms, due to differences in tasks and responsibilities as well as differences in pay for work of equal value (e.g. bargaining and discrimination). The remaining one quarter reflects differences in pay between firms, due to the concentration of women in low-wage firms. The gender wage gap within and between firms tends to increase over the working life. This reflects gender differences in opportunities for upward mobility between and within firms and the role of career breaks at the age of childbirth for the career progression of women. Tackling the gender wage gap crucially requires promoting access of women to high-wage jobs and firms.