Approximately one third of serious runners will incur an injury in a given year, and approximately one third of the injuries will involve the knee. Biomechanical studies on running reveal the tremendous cyclic forces to which the knee is subjected. There are several etiologic factors involved, including training errors, anatomic and biomechanical variations, and differences in shoes and running surfaces. Training errors, particularly rapid transitions in training, are responsible for two thirds of injuries. Knee injuries in runners must be approached with a thorough review of the training program and a complete examination of the lower extremity. Several conditions can cause anterior knee pain commonly involving the extensor mechanism, and these must be differentiated. Most knee injuries can be resolved with conservative treatment, but occasionally surgery is indicated. After a layoff due to injury or surgery, appropriate rehabilitation, including a graduated return to running, is essential to avoid reinjury.