This paper describes the transdiagnostic theory and application of family-based treatment (FBT) for children and adolescents with eating disorders. We review the fundamentals of FBT, a transdiagnostic theoretical model of FBT and the literature supporting its clinical application, adaptations across developmental stages and the diagnostic spectrum of eating disorders, and the strengths and challenges of this approach, including its suitability for youth. Finally, we report a case study of an adolescent female with eating disorder not otherwise specified (EDNOS) for whom FBT was effective. We conclude that FBT is a promising outpatient treatment for anorexia nervosa, bulimia nervosa, and their EDNOS variants. The transdiagnostic model of FBT posits that while the etiology of an eating disorder is unknown, the pathology affects the family and home environment in ways that inadvertently allow for symptom maintenance and progression. FBT directly targets and resolves family level variables, including secrecy, blame, internalization of illness, and extreme active or passive parental responses to the eating disorder. Future research will test these mechanisms, which are currently theoretical. (Contains 1 figure.)