The benefits of exercise to one's general health and well-being are well known. Many people continue to choose running as their exercise of choice. With the popularity of running, many chronic injuries occur because of overuse phenomena, improper training techniques, or a combination of the two. Stress fractures are only one example of chronic injuries that can occur secondary to these repetitive injuries. To understand the cause of running injuries, a basic understanding of the biomechanics of running is helpful. This article provides the reader with an introduction to the biomechanics of both walking and running, as running is a natural extension of walking, with significant differences. Normal and abnormal gait in walking has been studied in most detail; running has been studied to a lesser extent. Although running naturally occurs at a higher velocity than walking, the actual differentiation results from the float phase of running, where both feet are off of the ground. Sprinting is an extension of running, with increased velocity, but it likewise has significant biomechanical differences. It is the least studied type of gait for the obvious logistical reasons encountered with attempting to evaluate the most rapid form of gait while encumbered with measuring devices.