Anterior cruciate ligament (ACL) rupture is one of the most common sports-related knee injuries. Because it is such a significant injury - requiring a long period of rehabilitation before the injured person is ready to return to physical activity, the burden of injury is high. This can help to explain why so much of orthopaedic research is devoted to evaluating the outcomes of ACL injury. However, while there has been extensive study of short-term (up to 5 years after injury) outcomes, few studies have followed patients beyond 15 years after their ACL injury. This is important because long-term follow-up can provide key insights to guide the information provided to newly injured patients (e.g. to help set realistic expectations of what the injured person can expect of their knee function in the future), and help us evaluate the efficacy of previous treatments.