Congenital differences of the upper limb occur in approximately 0.16 to 0.18 percent of live births. These patients provide a unique challenge for the reconstructive hand surgeon. The correct and timely diagnosis of hand and upper limb congenital differences will lead to appropriate care and rehabilitation. The International Federation of Societies for Surgery of the Hand has classified congenital upper limb differences based on abnormalities of embryogenesis: failure of formation of parts, failure of differentiation of parts, duplication, overgrowth, undergrowth, constriction ring syndrome, and general skeletal abnormalities. This classification scheme is used as a basis for discussion of the most common upper limb anomalies. Both surgical and nonsurgical treatments are discussed, as is appropriate timing of intervention.