Authenticity has recently emerged as an important issue in discussions of mental disorder. We show, on the basis of personal accounts and empirical studies, that many people with psychological disorders are preoccupied with questions of authenticity. Most of the data considered in this paper are from studies of people with bipolar disorder and anorexia nervosa. We distinguish the various ways in which these people view the relationship between the disorder and their sense of their authentic self. We discuss the principal modern accounts of authenticity within the analytic philosophical tradition. We argue that accounts based on autonomous, or wholehearted, endorsement of personal characteristics fail to provide an adequate analysis of authenticity in the context of mental disorder. Significant elements of true self accounts of authenticity are required. The concept of authenticity is a basic one that can be of particular value, in the context of self-development, to people with mental disorder and to others experiencing substantial inner conflict.