The wages of part-time workers are considerably lower than are those of full-time workers. Measurable worker and job characteristics, including occupational skill requirements, account for much of the part-time penalty. Longitudinal analysis indicates that much of the remaining gap reflects worker heterogeneity, evidenced by small wage gains and losses among workers switching between part-time and full-time jobs. The lower skills of part-time than fulltime workers result primarily from limited work experience and accumulation of human capital. Little evidence can be found of a large wage gap between part-time and full-time women. A part-time wage penalty is found for men, but men account for a small proportion of total part-time employment.