The anterior cruciate ligament (ACL) is most commonly injured during sports-related activity, with a minority of ACL injuries occurring in high-energy trauma or activities of daily living. Approximately 70% of patients hear or feel a pop at the time of injury. Almost all patients notice swelling of the knee within 24 to 48 hours of the injury. Injuries may be classified as contact or noncontact injuries. Noncontact injuries usually occur with cutting or pivoting. Female athletes have a two- to four-fold higher risk of ACL injury than males when level of competition, age, and time exposed are taken into consideration. The reason for the higher rate of injuries among females is not clearly understood. Potential contributing factors include biomechanics, alignment, muscle strength, hormonal factors, and training.