The objective of this dissertation was to determine if the presence of patella alta influences knee extensor mechanics and patellofemoral joint stress. To achieve this goal three separate studies were undertaken. In the first, patellofemoral alignment and contact area were quantified in a group of persons with patella alta and compared to a control group. Additionally, the association between vertical patellar position, alignment and contact area was determined. Subjects with patella alta had greater malalignment and less contact area. There was a positive association between vertical patellar position and alignment and a negative association between vertical patellar position and contact area. In the second study, a detailed analysis of the knee extensor mechanism was conducted to test the hypothesis that persons with patella alta may be predisposed to elevated patellofemoral joint reaction forces. Four variables characterizing knee extensor mechanics were quantified in a group of persons with patella alta and compared to a control group. Contrary to the initial hypothesis, persons with patella alta had a more efficient quadriceps mechanism and were not predisposed to elevated patellofemoral joint reaction forces in the range of zero to sixty degrees of knee flexion. In the third study, an imaging based, biomechanical model of patellofemoral joint stress was developed to test the hypothesis that persons with patella alta would exhibit elevated patellofemoral joint stress compared to pain-free control subjects during gait. This model utilized subject specific knee extensor mechanics, patellofemoral joint contact area, knee kinematics and kinetics as input variables. Output variables were patellofemoral joint reaction force, utilized contact area, and stress. Persons with patella alta demonstrated significantly higher peak patellofemoral joint stress during fast gait compared to the control group. The cause of the elevated stress was the result of significant reductions in contact area as joint reaction forces were similar. Overall, the results of this dissertation support the premise that the presence of patella alta predisposes persons to greater patellofemoral joint stress during walking. The underlying mechanism appears to be related to alteration in the load-bearing surface of the patellofemoral joint as knee extensor mechanics are largely unaffected by this condition.