Treatment of anterior cruciate ligament injuries has changed considerably in recent years. The purpose of this paper is to discuss the past and present treatment for anterior cruciate ligament (ACL) disruptions in athletic individuals. In addition, this paper will discuss current trends in rehabilitation, such as immediate motion, weight bearing, and close kinetic chain exercises, and provide the scientific rationale for these rehabilitation principles. The treatment of individuals who have suffered an ACL disruption has changed dramatically over the years. The treatment of ACL ruptures has made a full circle. The first reconstructive procedure described used a patellar tendon graft. Then primary ACL repairs were advocated. With the limited success of that procedure, the nonoperative treatment was popularized, with reconstruction performed only after the conservative program failed. With this treatment plan, clinicians noted early degenerative joint changes and an increase in meniscus tears in the ACL deficient knee. Thus, reconstructive surgery using a patellar tendon graft was again advocated. Today, the current trend in the treatment of ACL tears is an arthroscopically assisted procedure to reconstruct the ACL using a bone-tendon-bone graft, such as a patellar tendon. The surgery employs accurate graft placement, tensioning, and fixation, which allows the therapist the opportunity to utilize immediate motion and weight bearing, in addition to strengthening exercises. This paper attempts to explain the rehabilitation process following ACL reconstruction using current scientific and clinical research. The program is based on the anatomy, biomechanics, and healing process of the knee, joint, and ACL. The clinical implications of this paper are numerous. First, we believe the information will assist clinicians in developing their own programs. Second, the data will assist the reader in understanding the sequential healing process. Finally, this paper documents that immediate aggressive rehabilitation is not deleterious to the ACL graft, and early therapy improves the functional outcome. J Orthop Sports Phys Ther 1992;15(6):279-293.