The kinematic variability of the upper extremity presents challenges when developing or analyzing surgical treatments of the shoulder and elbow. The wide range of possible activities places diverse loading conditions on these joints. To improve surgical outcomes at these joints the relationships between joint loading and kinematics must be established. In the shoulder, an optimally-based in vitro simulator of glenohumeral motion was developed. It was validated using simple humeral abduction activities and a mechanical surrogate shoulder. Our approach demonstrated the viability of our technique and showed how the use of the simulator could be expanded to cadaveric shoulders. In the elbow, computational models were coupled with motion analysis studies to determine the response of a contemporary total elbow replacement to loading induced by typical activities of daily living. Activities of daily living were determined from normal subjects and total elbow replacement patients. Differences between the kinematics and loading of the groups were statistically significant, although numerically small. A biomechanical model demonstrated that the contact force at the replaced elbow joint increased significantly as the humeral component of the elbow was internally rotated. It showed that these increased forces would occur in locations likely to cause additional further internal rotation of the component. Structural finite element analyses of both the humeral and ulnar bone implant systems illustrated the sensitivity of load transfer and cement stresses to the interface conditions between the implant and the cement layer. As implant-cement bonding decreased, load transfer away from the joint increased in both the humerus and nina. The mode of humeral load transfer was dependent upon the extent of contact between the implant and the distal humeral bone. Relaxing the bonding between the implant and the cement was shown to have a greater detrimental effect on the ulnar cement stresses than those of the humerus. These analyses all serve to illustrate the complex interplay between joint motion and kinetic and structural response that must be considered when investigating the behavior of the upper extremity.