During total hip arthroplasty, the biomechanics of the joint may be altered by removal of bone and by a change in the center of rotation of the joint. Joint pathologies existing at the time of reconstruction may also affect post-operative joint motion. In order to achieve optimized biomechanics of the replaced joint, it is important to understand the muscle actions that are involved in joint movement and the forces that are imposed on the construct by patient activity. To ensure survivorship of the replacement, intraoperative and long-term stability of the components making up the joint within host bone must be achieved. The patients receiving total hip replacements in the twenty first century tend to be younger, heavier, more active and longer lived than the patients who first received hip implants. Thus, biomechanical decisions are becoming even more important for long-term survivorship of the reconstruction.