A wide variety of clinical problems and relational styles are collected under the diagnostic heading of personality disorder (PD). These disorders involve maladaptive, persistent ways of thinking, feeling, and behaving that are associated with both functional impairment and disturbed interpersonal relationships. Personality disorders are difficult to treat, and challenge a therapist's ability to intervene helpfully, in part because the maladaptive patterns can impact the therapy relationship itself. Therapeutic principles were derived by Task Force review from the small body of research conducted to date with this family of disorders and include characteristics of clients and therapists, their relationship together, and essential elements of technique. The importance of a collaborative relationship as well as a treatment that is comprehensive, empathic, patiently applied, and flexibly tailored to presenting problems are dominant themes across principles. Treatment research with PD is in its early stages and many important areas remain uncharted. The Task Force principles are here summarized in hopes of providing general guidance to clinicians working with PD, as well as to outline research needs for the area.