Vascular malformations are developmental disorders involving the blood and lymphatic vasculature. The study of vascular malformations is a complex and rapidly evolving field with new understanding and insights into the clinical, histopathology, and molecular basis of these lesions. Diagnostic classification of vascular malformations is based on their unique clinical, radiologic imaging, histologic, and molecular characteristics. This article follows the published classification of the International Society for the Study of Vascular Anomalies. It describes characteristic histopathologic findings in tissues of vascular malformations and information on known genetic causes.