Proteus syndrome is a complex disorder consisting variably of disproportionate, asymmetric overgrowth of body parts; cerebriform connective tissue nevi; epidermal nevi; vascular malformations of the capillary, venous, and lymphatic types; and dysregulated adipose tissue. Serious complications may ensue, such as pulmonary embolism, cystic lung disease, and various neoplasms. Somatic mosaicism, lethal in the nonmosaic state, is the best working hypothesis. Although Proteus syndrome data are consistent with this hypothesis, it has not been proven. The etiology is unknown to date. Diagnostic criteria are emphasized because misdiagnosis of Proteus syndrome is common. Finally, evaluation and management are discussed. © 2005 Wiley-Liss, Inc.