The anterior cruciate ligament (ACL) is a crucial structure for a normal kinematic of the knee joint. Its rupture has been thought to present the initiation of a sequel leading to secondary damage of the meniscus and finally to osteoarthritis. Diagnosis of an ACL rupture is based on the history as well as on the physical examination. A positive Lachman-sign is diagnostic for an ACL lesion. The question whether an ACL rupture should be operated or not must be decided individually and is based on the demands of the patient. Conservative treatment includes rigorous physical therapy emphasizing strengthening exercises of the hamstrings and the gastrocnemius muscles as well as prorioceptive training. The indication for reconstruction is given, should instability persist. The timing of the operation is important for the outcome. This paper presents an overview of the literature including the rehabilitation program and a diagnostic concept used by the authors.