In this issue of Medical Clinics of North America, guest editors Drs. Heather Toth, Robert Kliegman, and Brett Bordini brings their considerable expertise to the topic of Adolescent Undiagnosed and Rare Diseases. Top experts address diagnostic dilemmas, disease mimics, unusual manifestations of common diseases, and rare disorders presenting as more common diseases in adolescents. Contains 15 relevant, practice-oriented topics including syndromes with severe acne; common variable immunodeficiency (CVID); overlapping pain syndrome; late onset congenital hypoventilation syndrome (LOCH); adolescent onset of muscle weakness; and more. Provides in-depth clinical reviews on adolescent undiagnosed and rare diseases, offering actionable insights for clinical practice. Presents the latest information on this timely, focused topic under the leadership of experienced editors in the field. Authors synthesize and distill the latest research and practice guidelines to create clinically significant, topic-based reviews.