This article concerns dual job holding and its link to job mobility. The authors present evidence from U.S. data on patterns of dual job holding, hours changes, and job mobility. They find that workers move into and out of second jobs frequently, that these movements are associated with large changes in work hours, and that hours constraints may prompt workers to take second jobs. Second, the authors review theories of dual job holding and present a stochastic dynamic model of dual job holding and job mobility in which decisions to take second jobs and/or change main jobs are made simultaneously. Copyright 1996 by University of Chicago Press.