Moonlighting by residents has been a major concern of residency program administrators, since extensive moonlighting by residents in addition to their already demanding residency work schedule might jeopardize patient care. In this paper, the authors examine the influence of several factors on residents' decisions to moonlight. The results indicate that the main motivating factor behind their moonlighting is economic and that residents moonlight more frequently when they have the opportunity to do so. In addition, the residents with higher levels of medical school indebtedness were significantly more likely to moonlight than residents with less indebtedness. Personal characteristics of the residents and their specialties also influenced moonlighting but had a smaller impact than amounts of stipends and indebtedness and the number of off-duty hours. The results also suggest that residency administrators can influence their residents' decisions to moonlight.