We study the part-time penalty. Using Danish register data, the Danish Labor Force Survey, and hospital personnel records, we show that the pay gap between part-time and full-time workers is sizable and increases over the career because the two groups accumulate different levels of human capital over time. Our best estimates of the part-time penalty are for nurses. The penalty is 14 percent at the beginning of the career and increases by 0.5 percent each year. This pay gap is closely related to the development of nurses' competence level, highlighting the persistent effects that part-time work has on lifetime earnings.