KLEIN--Donald F., M.D., who played a transformative role in psychiatry's evolution as a scientific discipline, died August 8, 2019. At the time of his death, Dr. Klein was Emeritus Professor of Psychiatry at the Columbia University Vagelos College of Physicians and Surgeons and the New York State Psychiatric Institute and Research Professor at New York University Langone Medical Center. Dr. Klein entered the field of psychiatry when its theoretical orientation and mode of clinical practice was transitioning from one dominated by psychoanalytic theory to psychopharmacology and evidenced based care. He was a major force in this process. In the early 1960's, he developed the concept of pharmacologic dissection of mental disorders using treatment response to distinguish subgroups within a diagnostic category. Over time, he became a highly respected and oft quoted researcher, educator and mentor who was known for his pithy comments and withering critiques. Donald Klein was born in 1928 in New York City, attended The Bronx High School of Science and graduated magna cum laude from Colby College. He received an MD from the SUNY College of Medicine (Downstate) in 1952, spent two years working at the U.S. Public Health Service in Lexington, KY and then completed a psychiatry residency at Creedmoor State Hospital. In 1959, he took a position at Hillside Hospital where he began his ground-breaking research on the diagnosis and treatment of mental disorders. This work led to the identification of distinct forms of anxiety and mood disorders and demonstrated their effective treatment with TCA and MAOI antidepressants. In doing so, he conducted some of the first clinical trials looking at combined effects of medication and different types of psychotherapy. He also co-authored, with John Davis, in 1969, the first clinical psychopharmacology textbook, "Diagnosis and Drug Treatment of Psychiatric Disorders." In 1976, Dr. Klein joined the faculty of Columbia University Department of Psychiatry and New York State Psychiatric Institute as Director of Research and Chief of the Division of Therapeutics where he oversaw the establishment of specialized research programs for the study of depression and anxiety disorders and pursued studies of their pathophysiology and treatment. In addition to his research, he gained a well-deserved reputation as a gifted clinician whose services were sought by numerous people including celebrities, political and business leaders. Dr. Klein served in many leadership positions including as past president of the American College of Neuropsychopharmacology (ACNP), the American Psychopathological Association (APPA), and the American Society of Clinical Psycho- pharmacology (ASCP), sci- entific advisor to the Alcohol, Drug Abuse, and Mental Health Administration (ADAMHA) and was a consultant to the Federal Drug Administration (FDA) Psychopharmacology Advisory Board. Throughout his career, he was a prolific writer and commentator on scientific and clinical matters of importance in psychiatry. In addition to his innovative research and clinical skills, Dr. Klein was a generative mentor who launched the successful careers of numerous proteges. He is mourned by his friends and colleagues and survived by his wife, the distinguished child psychologist, Dr. Rachel Klein, his daughters, and grand-children. From the Columbia University Vagelos College of Physicians and Surgeons and the New York State Psychiatric Institute Jeffrey Lieberman, M.D. Chair of Psychiatry