Clinicians and researchers have become increasingly aware of the important role of m echanical loading patterns in the onset and progression of knee joint abnormalities. Experimental and clinical studies of the etiology and progression of these disorders require three-dimensional m odeling of the knee joint, including an analysis of the frontal plane, for determ ination of tibiofem oral joint contact forces. Knowledge of the load distribution between the tibial plateaus may aid in identification of biomechanical factors that contribute to angular joint deform ities or osteoarthritis, and may help in the design of movement modifications or clinical treatments to reduce these effects. The goals of this dissertation were to investigate the knee joint biomechanics in a common experimental animal model and in children who are overweight, and therefore at risk for knee joint disorders. Force platform data and three-dim ensional m otion analyses using infrared m arkers were combined to calculate the lower limb intersegmental forces and moments during hopping in adult rabbits and normal gait of children of varying weights. A statically determ inate model was developed to predict muscle, ligament and tibiofemoral joint contact forces during the stance phase of gait. In the rabbit, the prevalence of an internal knee adduction moment led to the prediction of higher joint contact forces passing through the lateral compartment of the joint, which is contrary to what is expected in the human knee. These data provide an improved understanding of the use of the rabbit model for investigations of knee joint disorders. Children who are overweight showed evidence of gait adaptations to maintain similar knee extensor loads to children of normal weight, however this ability to compensate was reduced at faster speeds. In addition, at both speeds the children who are overweight walked with increased internal knee abduction moments, leading to predictions of increased medial com partm ent joint contact forces, sufficient to suggest increased risk for trabecular bone fatigue damage. In summary, these results represent critical data on the biomechanics of the knee joint in both rabbits and children, thereby providing further insight into the relationships between m echanical loading patterns and the developm ent of knee joint disorders, both clinically and experimentally