The anterior cruciate ligament (ACL) has a mechanical and a sensory function during daily activities and during strenuous activity, such as sport. There are 2,500 ACL ruptures/year in Denmark, mostly occurring during sports. Usually the diagnosis can be established clinically. Acute suture of the ligament gives unsatisfactory results, but reconstruction provides mechanical stability and good knee function in 80%. Less than 50% are able to return to top level sports. The long-term risk of degenerative changes in the knee is increased 10-15 fold after an isolated ACL rupture. Untreated ACL ruptures have a particularly poor prognosis in children. Proprioceptive training can reduce the risk of ACL injuries in football. There is little evidence in support of treatment strategies with respect to surgery and rehabilitation after ACL injury. Randomized studies should be initiated, and in Denmark epidemiological data on ACL injuries and treatment could be registered in a national database.