Over the past few decades, pro bono practice has undergone dramatic changes in how it is structured and has increasingly become institutionalized throughout the legal profession, especially across large law firms and law schools. In this study, I employ both quantitative and qualitative data to examine the institutionalization of pro bono in large elite law firms along two dimensions. First, I analyze large firm participation in terms of hours between 1993 and 2005 for the largest 200 firms in the United States. Specifically, I examine a variety of organizational and institutional factors that predict how much time a firm commits to its pro bono practice. Second, I analyze the scope of pro bono representation through a cross-sectional analysis of the inter-organizational relationships between a sample of large firms and their pro bono clients. My quantitative analyses are supplemented with interviews of pro bono coordinators across the county in order to contextualize contemporary pro bono practices. These interviews provide insights into how law firm actors structure their pro bono practice and the processes by which they decide to work with specific organizations. My findings challenge the functionalist imagery of pro bono, which is so prevalent in most discussions surrounding the topic. Pro bono is often thought to derive from the interests of different actors, including law students and corporate clients. Instead, my findings suggest that pro bono is best conceived as an institutional project that is driven not by the demands of interested actors but through social factors. In my quantitative analyses, I find that institutional factors are more important than organizational factors (e.g., profits per partner or number of lawyers) in predicting the amount of time that a firm commits to pro bono. Qualitative evidence supports these findings and indicates that the rise of pro bono ranking systems and competition with peer firms are more important factors driving how firms think about pro bono.