Radial and ulnar longitudinal deficiencies are the 2 most common types of congenital longitudinal deficiencies of the arm, with radial deficiency being 3 to 4 times more common. They are a spectrum of abnormalities, ranging from mild deficiency of the digits to complete loss of one-half the forearm, wrist, and fingers. Radial longitudinal deficiency is associated with a number of medical syndromes that require a comprehensive medical evaluation, while ulnar longitudinal deficiency (ULD) is associated with other musculoskeletal anomalies. Both conditions have a high incidence of ipsilateral thumb abnormalities. Wrist and forearm procedures, such as soft tissue distraction and centralization, are more often required in radial longitudinal deficiencies than in ULD. Elbow involvement can occur in both conditions but is more frequent and often more severe in ULD.