 A total of 240 patients will be enrolled across participating sites around the country. These sites currently include: The Hospital for Special Surgery, Oregon Health and Science University, Inova Sports Medicine, and Mayo Clinic. Patients are randomly assigned to one of two treatment groups. Patients assigned to the stabilization group will undergo surgery to reconstruct the MPFL using either a hamstring tendon that is your own or one obtained from a donor. Patients assigned to the conservative group will be treated with physical therapy, and possibly a simple knee arthroscopy if you are found to have a loose fragment on x-ray or MRI. All patients will be required to return for follow-up visits, which will continue over the course of 5 years. During these visits, patients will be examined, asked to answer questions about their surgery and general health, and have imaging x-ray and MRI performed on the knee. All patients enrolled in this study will undergo a standardized, intensive rehabilitation protocol. The investigators anticipate that all patients may benefit from extensive rehabilitation, including strengthening and return to sport training. The knowledge gained from this study may benefit others in the future. 