Although the results of TKR are highly successful at long-term follow-up, failures occur. One of the more frequent causes of failure is instability. In distinction to instability in the medial-lateral plane, AP instability in flexion has been poorly described until recently. Although acquired ligamentous incompetence can occur, particularly with cruciate retaining prostheses, many cases of flexion instability result from an intraoperative failure to create symmetric balanced flexion and extension spaces. In primary TKR, use of a well-designed posterior stabilized prosthesis and creation of symmetric balanced flexion and extension gaps should minimize the incidence of postoperative flexion instability. If flexion instability occurs, the role of nonoperative treatment is limited. In most cases, revision TKR using the same basic principles is required. When symmetric flexion and extension spaces cannot be produced intraoperatively in complex primary or revision surgery, use of a more constrained articulation, such as a constrained condylar prosthesis or hinged prosthesis, is required.