Hemangiomas of infancy are unique, benign, pediatric tumors of endothelial cells characterized by an initial phase of rapid proliferation, followed by slow involution, often leading to complete regression. Although most of these tumors are small and innocuous, some may be may be life- or function-threatening, or have associated structural congenital anomalies. Uncertainties regarding their diagnosis or management often prompt referral to a dermatologist. The pathogenesis of hemangiomas of infancy is not well understood, but recent findings suggest a unique vascular phenotype with dysregulated vascular homeostasis. This article reviews new information regarding the pathogenesis of these tumors and highlights the more worrisome presentations, including syndromic hemangiomas, that are likely to be problematic. In addition, management strategies and treatment options are discussed. (J Am Acad Dermatol 2003;48:477-93.) Learning objective: At the completion of this learning activity, participants should be able to describe the clinical features of hemangiomas of infancy and potential complications as well as to understand the strengths and limitations of various treatment options.