This is the fourth and final article in a special series on personality disorders (PDs). The focus of this article is the course and stability of PDs. Studies that have addressed the naturalistic history and course of PDs are reviewed. Selected findings from treatment outcome studies that have implications for understanding the course of PDs are briefly presented. Based on available evidence, we conclude that 1) PDs demonstrate only moderate stability; 2) although generally associated with a plethora of negative outcomes, PDs can show improvement over time; and 3) psychosocial and pharmacologic interventions can have a positive impact on the course of PDs. This review highlights more questions than answers. We suggest implications for future research and note methodologic issues that must be considered in order to determine whether PDs are, in fact, enduring and stable patterns. An improved understanding of the course and stability of PDs is needed in order to inform our diagnostic system(s).