Participants will enroll in this 12-year study 1 to 6 weeks prior to ACL surgery. There will be two preoperative study visits: one will include magnetic resonance imaging (MRI) and the other will include a knee evaluation, dynamic function testing, and questionnaires. Postoperative visits occurred immediately following surgery and at 6, 12, 36, 60, 84, 120 and 144 months following surgery. Strength testing, functional testing, x-rays, questionnaires, and a knee exam will occur at most postoperative visits. MRIs will occur at some postoperative visits. An additional group of participants with no evidence of knee injury will serve as a control. The control group will attend all study visits except for the 12-month visit. All participants may be followed for up to 10 years.