Many runners sustain repeated injuries on the same side of their bodies. This suggests that they may be particularly susceptible to injury on a given side. Injury risk on a particular side of the body may be related to asymmetry. The overall purpose of this work was to improve the methods of gait symmetry collection and quantification and to elucidate the role of side-to-side differences within runners that predispose some to overuse injury. This was addressed in four studies. The first study focused upon the formulation of a new method, the Symmetry Angle, for quantifying symmetry between discrete values. The new method is robust to some limitations of the previously-used Symmetry Index, such as the requirement of a reference value and artificial inflation. The second and third studies addressed the bilateral factors related to why runners become injured, often repeatedly, on a given side. The second study examined runners with previous overuse injuries, and compared them to runners who had never sustained a running injury. The results identified three variables: hip internal rotation range of motion, peak tibial shock, and deviation from normal arch structure, that seem to be abnormal in the injured runners. The third study examined runners who sustained an overuse injury in the nine months following the data collection, and compared them to runners who remained uninjured during that period. Descriptive results of the prospective study did not show strong support for imbalances between the injured and uninjured sides of injured runners, or for bilateral abnormalities between injured and uninjured runners. However, like the retrospective study, asymmetry levels were similar between injured and uninjured runners. The final study compared gait asymmetry levels measured from consecutive and non-consecutive footstrikes. The results of this work suggest that gait characteristics tend to be sufficiently consistent within a side, such that differences between sides can be reliably assessed using either consecutive or non-consecutive footstrikes. This work may be particularly useful for validating the results of previous studies where non-consecutive footstrikes have been used, as well as for subsequent research to be conducted in laboratories where only one force platform is available.