Lower limb loss continues to be a major disability with serious psychological, sociological and economic consequences for the amputee. Improving amputee mobility through the design of improved prostheses requires an increased understanding of the gait process. The general goal of this work is to advance fundamental understanding of the biomechanics of amputee level walking and to utilize these insights to determine the optimum characteristics of prostheses. Dynamic mathematical models of above-knee amputee gait will be developed to better understand the prosthetic gait process. The simulation models are intended to: 1) describe which force inputs and amputee/prosthesis parameters dominate stance phase mechanics; 2) predict how changes in these inputs and parameters affect gait mechanics; and thereby 3) determine design specifications for improved above-knee prostheses. First, a set of inputs representing the amputee's intent to control the prosthesis will be determined. Second, the model will use these inputs to predict the new gait mechanics that result from changes in prosthesis function. This work will aid in the design of improved above-knee prosthesis by quantifying the dynamic interdependence of the elements in the amputee/prosthesis system. The results will also be valuable in the design of other types of leg prostheses and lower limb orthoses. Some basic differences between normal and prosthetic gait will be quantified.