How many and which individuals, with which psychiatric disorders, receive (and do not receive) mental health services from which professionals in what settings? This question falls within the purview of mental health services research, which is a multidisciplinary field that brings together the methodologies of epidemiology, econometrics, and clinical research. First, in this article, we present an explication of what is known about those individuals in need of psychotherapy and how they access services. Next, we describe the numbers, professional affiliations, and service sites of professionals who are engaged in the practice of psychotherapy. We summarize our current knowledge about the actual utilization of psychotherapy services relative to the needs of patients and the professional background of therapists. Finally, we identify aspects of psychotherapy service utilization that are, as yet, unaddressed.