A study examined the characteristics of moonlighters and the length of their moonlighting episodes to determine who moonlights and why. Data for 203 prime-aged men were drawn from the 1984 Survey of Income and Program Participation panel (SIPP). The methodology involved a study of the personal and job-related characteristics of moonlighters and how the length of the moonlighting episode varied with these characteristics and an estimation of a duration model with unobserved heterogeneity to identify formally the determinants of moonlighting behavior when multiple motives may exist. The expectation was that individuals who moonlighted because they were constrained on their primary jobs might do so for shorter periods than those who moonlighted for other reasons. Therefore, the hazard rate for workers who moonlight because of primary job constraints should be greater than for those with alternative motives. The descriptive analysis revealed that most moonlighters worked full time on their primary jobs and 15-20 hours per week on lower-paying second jobs, and, in spite of those long hours, tended to be poorer than the average workers. Yet, a significant minority earned a higher wage on their second job. The duration model results suggested the structural hazard increased over time and there was significant unobserved heterogeneity. These results were consistent with the presence of multiple motives for moonlighting, with the constraint motive being the most common. (Contains 19 references and a technical appendix.) (YLB)