Venous malformations are the most common vascular anomalies. When they are superficial and large, they are easily recognized. However, when the malformation is deep or involves normally occurring veins, the diagnosis can be challenging. Extremity varicosities may be the only visible signs of the problem. Unfortunately, varicosities are often presumed to be the result of superficial venous insufficiency and the underlying venous malformation is overlooked. The phlebologist must be cognizant of the possibility that a venous malformation may be the cause of the patient's symptoms and varicosities. Ultrasound and magnetic resonance imaging can help to confirm the correct diagnosis. Treatment involves image-guided endovenous occlusion of the malformation, most commonly with potent liquid sclerosing agents. Some of these agents can cause severe complications. A thorough knowledge of the sclerosing solutions and the endovascular techniques to properly deliver them to the malformation is essential. This article will review the basic principle surrounding endovascular management of venous malformations.