The congenital deformities that affect the thumb are reviewed in terms of the clinical problem, the type of inheritance involved, the associated abnormalities seen with the particular deformity, and the indications for surgical treatment. The thumb is often the key to establishing the diagnosis of many rare syndromes. The congenital thumb may be classified for easy identification by pediatricians, family doctors, and hand surgeons as follows: inadequate thumb, fingerlike thumb, contracted thumb, crooked thumb, extra thumb, and unusual thumb.