This chapter reviews the DSM-5 system for the diagnosis of mental disorders, with an emphasis on its limitations and the danger of concept creep. Some alternatives to DSM have been proposed, but they do not have enough empirical support to replace that system. The chapter also considers how the same issues arise when using the International Classification of Diseases developed by the World Health Organization. The problems of overdiagnosis in psychiatry are reviewed with specific reference to major depression, bipolar disorder, attention-deficit hyperactivity disorder, autism spectrum disorders, and post-traumatic stress disorder. The issue of diagnostic inflation, related to concept creep, is examined, and the reasons why expansion of diagnoses has been so attractive to clinicians are discussed.