Upper limb defects occur in approximately 3.4 per 10,000 live births. Major thumb defects represent 16% of these upper limb defects (Tay SC, Moran SL, Shin AY, et al. The hypoplastic thumb. J Am Acad Orthop Surg 2006;14:354-366). Embryologically, hand development begins by the fifth week. This occurs simultaneously with the growth and development of the cardiovascular, neurologic, and hematopoietic systems. Therefore, congenital anomalies seen in the hands of infants may indicate significant anomalies in these other systems, requiring a comprehensive physical evaluation. Although the cause of 40% to 50% of congenital hand anomalies is unknown (Gallant GG, Bora FW. Congenital deformities of the upper extremity. J Am Acad Orthop Surg 1996;4:163-171), several others have traced this to specific genetic mutations. Others are due to a variety of teratogenic effects (Sadler TW. Langman's Medical Embryology. 10th ed. Philadelphia: Lippincott Williams &Wilkins, Chapter 9, 2006:125-142). For the clinician, this paper has been organized to identify possible corresponding syndromes that may accompany specific thumb deformities.