Runners are most susceptible to injury when they first begin a training regimen; when they return to training following a previous injury; when they increase their training volume (i.e., run a greater distance); or when they increase the intensity of their training (i.e., run faster). In most cases, the factors that cause running injuries can be identified and avoided. The most common cause of running injury is training errors, which may include inadequate stretching, insufficient rest between hard workouts, and sudden increases in mileage, hill training, or speed training. Other factors include inadequate footwear—that is, shoes that are too worn or are not appropriate for one’s foot type or training needs—or anatomic abnormalities. Common sites for running-related injuries include the groin, back, knee, hamstrings, quadriceps (thighs), ankles, shins, calves, and feet. In general, running injuries can be treated with a combination of rest, training modification, and rehabilitation. Physical therapy can help runners correct muscle strength, flexibility, and stability issues. In some cases, runners may need to change their training surface. The ideal training surface is flat, smooth, and soft; excessive mileage on hard surfaces such as concrete may lead to injury. Some runners—for example, those with extremely high arches—may benefit from use of orthotics or other shoe modifications.