Background: Volunteer physicians are crucial for the operation of safety-net clinics, which provide medical care for uninsured and underinsured populations. Thus, identifying ways to maximize the number of physicians volunteering at such clinics is an important goal.Objective: To investigate the perceptions, motivations, functions, and barriers associated with physician volunteering in four safety-net clinics in San Bernardino County, Southern California, a location of great medical need with many barriers to care.Methods: The study participants are physicians belonging to the Southern California Permanente Medical Group who use a combination of discretionary time (during regular work hours) and personal time in evening and weekend hours to volunteer their services. The experimental design incorporates a mixed methodology: an online survey of 31 physicians and follow-up interviews with 8 of them.Results: Physicians conveyed uniformly positive perceptions of their volunteer service, and most were motivated by humanitarian or prosocial desires. Volunteering also provided a protective "escape hatch" from the pressures of the physicians' regular jobs. Physicians cited few challenges to volunteering. The most common personal barrier was a lack of time. The most common professional barriers were organizational and supply issues at the clinic, along with the patients' social, transportation, and financial challenges. Conclusion:The results suggest that appealing to physicians' values and faith, and highlighting the burnout-prevention qualities of volunteering, may be key to recruitment and retention of volunteer physicians who serve underserved and underinsured populations in community clinics.