Adjustment disorder is a diagnosis that is commonly used, particularly in primary care and general medical settings. However, there has been relatively little research done on this disorder. In this article, the author reviews the information that is available on the epidemiology, clinical features, validity, measurement, and treatment of adjustment disorder. She first reviews the historical development of the diagnosis from transient situational personality disorder in DSM-I to its current definition in DSM-IV. The author also considers similarities and differences in how adjustment disorder is defined in the DSM and ICD systems. The clinical features of the disorder that distinguish it from disorders such as major depressive disorder, generalized anxiety disorder, posttraumatic stress disorder, and acute stress disorder are described. The author highlights a number of the common controversies concerning adjustment disorder, especially criticisms that the diagnostic criteria are often poorly applied and that the disorder itself involves the medicalizing of problems of living. Evidence in support of the validity of the adjustment disorder diagnosis is reviewed and the author concludes that the findings support the content and predictive validity of the diagnosis. The author then discusses the epidemiology of adjustment disorders, their comorbidity with other conditions, including personality disorders, substance abuse, and suicidal behavior, and their treatment and outcome. The article concludes with a discussion of the special problems involved in evaluating for and measuring adjustment disorder.