Explanations of altruism remain fragmented across disciplinary lines and focus heavily on phenomena such as philanthropy, the nonprofit sector, and volunteering outside the workplace. Yet numerous professions, including law, claim a duty of service that calls on their members to volunteer. Using a mixed methods approach that draws on thirty interviews and a survey of 845 lawyers, the authors develop an integrated framework on altruism to account for how volunteering takes place in the course of law practice. The analysis reveals psychological traits, collective norms, exchange relationships, and organizational dimensions that shape lawyers’ volunteering. In particular, a cultural norm endorsing volunteer efforts is a powerful driver of volunteering legal services. At the same time, organizational features, such as time constraints, condition cultural norms to hinder volunteering, while business opportunities for client recruitment condition cultural norms to foster volunteering. We conclude with directions for advancing our integrated approach to altruism in the context of lawyers’ professional service.