Varicose veins—gnarled, dilated blood vessels that occur most frequently in the legs—affect about half of the adult population of the United States. They are generally caused by pressure in the veins that, over time, weakens and damages the vessel valves and walls. For most people, varicose veins and spider veins (their smaller, capillary-based cousins) are merely unsightly nuisances, but they can also make walking and standing extremely painful, and in extreme cases, they can cause skin ulcers. While there is no sure way to prevent the formation of varicose veins, there are a number of things you can do to decrease your chances of getting them.