This review presents the current concepts in the treatment of anterior cruciate ligament (ACL) injuries. Conservative treatment emphasising strengthening of the muscles to stabilise an unstable knee is discussed. The agonistic and antagonistic functions of the hamstrings and the quadriceps muscle in relation to the ACL are described. The possible mechanisms of muscular stabilisation of the knee are discussed on a neurophysiological basis. The existence of joint specific receptors is pointed out and their reflex and perceptive functions are demonstrated. Taking this into account one can deduce a two-step mechanism of joint protection: 1) via the mechanical strength of the joint capsule and ligaments and 2) via reflex muscle contractions. It follows therefrom that in an unstable knee with a lax capsule and disturbed reflex mechanisms, strengthening of muscles alone is insufficient and cannot protect the joint from progressive deterioration. Therefore, surgical treatment is necessary for an ACL deficient knee. Our indications for ACL reconstruction are described. In addition, the existing methods of surgical replacement of the ACL are critically elucidated. In our opinion, an extraarticular repair tendon transfer as the only surgical procedure to regain stability is of only historical interest. However, extraarticular repair of the medial or lateral capsule is often a necessary additional step to restore stability of the knee. The replacement of the ACL is the crucial step. Using autogenous grafts one must consider the doubtful outcome of biological response leading to necrosis and revascularization in the transplant.(ABSTRACT TRUNCATED AT 250 WORDS)