The U.S. mental health (MH) workforce is comprised of core disciplines: psychology, psychiatry, social work, psychiatric nursing, and marriage and family therapy. A broader group of practitioners also deserves recognition. Diverse professions provide significant services in a variety of settings, extending the de facto mental health workforce. A tally of key disciplines estimates there are 537,857 MH professionals, or 182 per 100,000 U.S. population. This article provides an overview of the need and demand for mental health services and summarizes the MH professions (e.g., training, educational credentials, workforce estimates). It also discusses a range of challenges confronting MH professionals and the need for greater understanding of the workforce and integration of services. Methodological factors that confound estimates of the magnitude of the MH workforce are reviewed.