This chapter reviews the research literature on professional service firm–client relationships. Client relationships are a defining feature of professional service firms, but are also critical strategic assets and an indicator of the market strategy of professional service firms. To assess the state of knowledge about client relationships the chapter is organized around three themes. First is the different ways that professional service firm–client relationships have been characterized and how this shapes researchers’ assumptions about and focus on client relationships. Second, it examines the life cycle of client relationships, focusing specifically on research that addresses the formation, maintenance, and dissolution of client relationships. The third theme examines research that has identified how client relationships affect professional service firms, particularly in the areas of strategy, structure, learning, and human resource practices. The author identifies important gaps in our understanding of client relationships and suggests several potential avenues for future research.