The most common complication of anterior cruciate ligament (ACL) reconstruction is loss of extension, which is often functionally worse for patients than their preoperative instability. Many preventable surgical and nonsurgical etiologic factors have been identified. Accurate placement of the tibial tunnel, adequate notchplasty, and the routing of the femoral side of the graft are all critical factors. Several studies report that early range-of-motion therapy emphasizing immediate postoperative "hyperextension" and avoiding immobilization in flexion reduces the rate of loss of extension. Initial studies investigating the effect of acute versus chronic ACL reconstruction suggested that acute reconstruction is associated with a higher rate of loss of extension. However, the authors of two recent studies in which modern techniques were used have disputed this conclusion. It is likely that the loss of extension historically seen with acute ACL reconstructions was related to tibial tunnel placement and postoperative immobilization. It is possible that the timing of acute ACL reconstruction has less of an effect than originally postulated. On the basis of the results of several biomechanical studies, it appears that ACL reconstruction may be performed with the knee in full extension during graft placement with excellent results and a very low rate of loss of extension. Use of the descriptive term "loss of extension" is preferred to the often misleading terms "arthrofibrosis" and "flexion contracture."