Vascular anomalies affect up to 10% of newborns, largely because of the high incidence of hemangioma of infancy. Vascular anomalies also frequently occur in adults; there is high prevalence of capillary malformations (0.3%). These cutaneous stains often cause psychosocial problems related to their visibility. Venous malformations occur in the skin and in internal organs and may cause destruction. Primary lymphedema causes lifelong morbidity, and arteriovenous malformations, in addition to causing distortion, obstruction, and pain, can be life endangering. The pathophysiology of these anomalies has stayed largely unknown, but genetic studies have revealed clues to their etiology. Genetic defects cause hereditary types of venous malformation, cutaneous and mucosal (VMCM); glomuvenous malformation (GVM); primary congenital lymphedema (Milroy disease); lymphedema-distichiasis syndrome; hypotrichosis-lymphedema-telangiectasia (HLT) syndrome; hereditary hemorrhagic telangiectasia (HHT); cerebral cavernous malformation (CCM); and a newly recognized disorder, capillary malformation–arteriovenous malformation (CM-AVM). These seminal discoveries have not only permitted a more precise clinical classification and diagnosis (a prerequisite for corrective measures for prevention, treatment, and follow-up) but also pointed the way to the identification of factors that play an important role in vasculogenesis or angiogenesis, or both.