The permeability of the work/retirement boundary is examined by investigating the labor force reentry process among a group of male retirees. Using data from the National Longitudinal Survey of Older Men, hazards models are estimated to identify the determinants of postretirement work. Reentry is distinguished according to part-time and full-time work to capture potentially important sources of diversity. The results show that reentry occurs quickly - typically within the first year or two after a labor force exit. Despite career interruption, several work career factors significantly alter the overall chances of reentry. Comparing reentry determinants of full-time versus part-time work suggests that postretirement, part-time work is a distinct state referencing partial retirement; it is not a middle ground on a continuum between career work and complete retirement. In addition, characteristics positively linked to an initial early retirement negatively affect reentry into full-time (although not part-time) work. The determinants governing moves into and out of the labor force, and into and out of partial retirement are quite different. Overall, the results demonstrate that the transition from work to retirement is neither uniform nor irreversible.