In recent years there has been widespread acceptance that cognitive behavior therapy (CBT) is the treatment of choice for bulimia nervosa. The cognitive behavioral treatment of bulimia nervosa (CBT-BN) was first described in 1981. Over the past decades the theory and treatment have evolved in response to a variety of challenges. The treatment has been adapted to make it suitable for all forms of eating disorder--thereby making it "transdiagnostic" in its scope--and treatment procedures have been refined to improve outcome. The new version of the treatment, termed "enhanced CBT" (CBT-E) also addresses psychopathological processes "external" to the eating disorder, which, in certain subgroups of patients, interact with the disorder itself. In this paper we discuss how the development of this broader theory and treatment arose from focusing on those patients who did not respond well to earlier versions of the treatment. (Contains 2 figures and 1 table.)