Varicose veins are swollen, twisted veins beneath the skin, most often in the legs and feet. They can be painful and a source of embarrassment. The cause is a problem with the one-way valves inside the veins that help control blood flow back to the heart. If the valves are weak or damaged, blood will pool in the veins causing them to swell and stretch. As varicose veins progress, they may be painful and cause aches, cramping, and a heavy feeling in the legs.