The long term results of total knee replacement are among the most successful of any orthopedic procedure; however, certain patterns of failure have been identified including varus-valgus instability and anterior-posterior instability in flexion. While trauma may cause acute ligamentous injury that results in instability, the majority of cases likely result from intraoperative decisions that result in malalignment and inadequate soft tissue balancing. Failure to restore medial-lateral soft tissue balance with symmetric flexion and extension spaces may result in postoperative instability. Techniques to achieve correct soft tissue balancing and symmetric flexion and extension gaps, as well as the management of postoperative instability, are reviewed.