Total hip arthroplasty (THA) is the treatment of choice to relieve joint pain and loss of mobility as a result of advanced stage osteoarthritis or other hip pathologies. Despite their general success, THAs do fail, with revision rates estimated near 5% per year. Instability, defined as the complete subluxation (dislocation) of the femoral head from the acetabular socket – which usually occurs due to implant impingement – has recently supplanted wear-induced osteolytic aseptic loosening as the leading cause of failure in THA. Soft tissue integrity has long been recognized as influencing joint stability, and therefore there has been great interest recently in improving soft tissue restoration following THA. However, there is little quantitative information related to the degree of soft tissue repair necessary to restore joint stability. Additionally, impingement events, besides their role in prelude to frank dislocation, hold potential to damage newgeneration hard-on-hard bearings, due to the relatively unforgiving nature of the materials and designs. Despite the largely biomechanical nature of these impingement-related complications, they remain under-investigated relative to their burden of morbidity. In addition to impingement, failure modalities unique to hard-on-hard bearings merit careful biomechanical scrutiny. This includes investigation of catastrophic fracture in ceramicon-ceramic bearings, as well as analysis of patient, implant and surgical variables associated with increased wear and adverse soft tissue engagement potential for metalon-metal implants. Toward the goal of improving current biomechanical understanding of failure modalities in THA and to provide an objective basis for orthopaedic surgeons to choose the most favorable implants and to identify optimal intraoperative parameters which minimize failure propensity, a novel, anatomically-grounded finite element (FE) model with hip capsule soft tissue representation was developed. This FE model was used to investigate four principal modes of failure in THA, including dislocation, impingement, fracture mechanics of ceramic implants, and various issues related to failure mechanisms in metal-on-metal implants. The influence of soft tissue integrity and patient obesity on dislocation was investigated. The model demonstrated that (1) posteriorly directed capsule defects resulted in a substantial decrease in THA stability; (2) proper repair of these defects returned stability to near baseline levels; (3) repairs with too few sutures risked suture failure; (4) dislocation risk in obese patients increased for body mass index exceeding 40 kg/m²; and (5) dislocation risk in obese patients could be reduced with the use of cups in lower degrees of inclination and with the use of an offset femoral neck. The FE model was also used to investigate potentially deleterious consequences of impingement in THA. It was determined that (1) egress site stresses exceeded impingement site stresses; and (2) stresses generated from bone-on-bone impingement were less severe than those from hardware impingement scenarios. Linear elastic fracture mechanics FE and eXtended FE (XFEM) models were also developed to investigate fracture risk in ceramic liners. Fracture risk was found to be increased (1) for malpositioned cups; (2) during stooping and squatting motions; (3) for cups with sharp edges; and (4) for instances of high body weight. The final section of the research involved various failure modes using metal-on-metal implants. Edge-loading is a particularly important consideration for metal-on-metal THA, and edge-loading severity was found to be highly sensitive to subtle changes in cup design. Additionally, a novel method of implant orientation optimization was developed, and allows for ideal acetabular cup positioning to be determined for any femoral head size and any patientspecific degree of femoral anteversion. Finally, wear potential at the trunnion interface for large diameter THA was found to increase dramatically for head diameters exceeding 40mm.