While the majority of gait research is focused on flat, level surfaces, in everyday life individuals must navigate diverse and changing terrain. Thus it is important to investigate gait patterns on multiple surfaces, which is why this dissertation concentrates on hill walking and running as well as the transitions between level and hills. To this end, it was necessary to create a versatile, kinematic-based event detection algorithm that was capable of detecting foot strike and toe off times for multiple gaits at various surface angles. Besides detailing the specific strategies used, we sought to understand how physiological and environmental factors influence the strategies that individuals choose. Many researchers have suggested that the preferred stride frequency (PSF) is determined by mechanical factors. However, this view ignores muscles which are the primary consumers of metabolic energy during running. Thus, we investigated the role of muscle activity in influencing the PSF during downhill running. We found that the PSF may be determined by a tradeoff between the cost of muscle activity in the stance phase, which is greater at slower frequencies, and muscle activity in the swing phase, which is greater at faster frequencies. This balance would also explain why the PSF minimizes metabolic cost. It has also been shown that segment lengths affect level walking. Specifically, at the same speed, taller individuals take longer, slower steps. However, there have been no studies that investigate the influence of segment lengths on meeting the differing demands of hill walking. We found that during hill walking, muscle activity increases with decreasing shank length. This is likely due to either a smaller muscle compartment containing a smaller muscle which requires greater activation to produce the required force, or is related to shorter individuals walking at a relatively faster speed compared to their preferred speed. Older adults typically walk with a cautious gait pattern characterized by shorter steps, slower speeds, and wider base of support. In addition, age related loss of muscle mass and control result in weakness, especially in the ankle extensors which leads to additional gait modifications and compensations. Transitioning onto ramps has been shown to increase the risk of falling, and, in response, young adults employ similar cautious gait patterns. Thus, we wanted to determine how older adults, who are weaker and already cautious, handle the task of transitioning onto and off of ramps. We found that older adults exaggerate the transitions maintaining a cautious pattern, but modifying it in order to meet the demands of the transition. Besides greater fall risk, we found that during transition steps, individuals anticipate the demands of the future surface. However, we do not know what features of the ramp—the angle, length, or plateau height—influence our perception of the demands and ultimately what transition strategy we choose. We found that, though there are some modifications related to the total amount of work to be done, i.e. the length and height of the ramp, the angle of the ramp is the primary factor influencing the transition strategy. Overall, by focusing on the relationship between locomotor strategies and the factors that influence them, we hope to improve diagnostics and rehabilitation by approaching it as a matter of manipulating these factors through strength training or environment design.