Congenital limb duplications include pre- and post-axial polydactyly, central polydactyly, and the mirror-hand spectrum. Treatment of these duplications constitutes a significant functional and aesthetic challenge for the reconstructive hand surgeon. This article provides an inclusive review of the embryologic and molecular mechanisms underlying these deformities and focuses on their clinical treatment. The anatomic variation, classification, surgical treatment, and outcomes of surgical intervention are reviewed for each of the disorders of duplication.