The shape of your foot is straight, semi-curved or curved. To determine which, stand on a piece of paper and trace the outline of each foot. Draw a straight line from your heel to your toes. If the line runs through your first two toes, you have a straight foot. The farther away from the big toe it runs, the more curved your foot is.
Many people overpronate when they run, which means the feet roll inward and flatten out.
Less common are people who oversupinate, which means the feet roll through the ball and rotate outward. If you overpronate, look for a shoe with plenty of motion control. Straight-lasted shoes are usually best, too. Those who oversupinate should opt for flexible, well-cushioned shoes. When trying on shoes, wear the socks you normally use when working out.