Assessment of the natural history of various knee injuries and the effects of operative and nonoperative treatments requires validated outcome measures. We present and review various outcome measures for the knee, including subjective patient reporting systems, comprehensive patient and clinician instruments, and activity scales. The validated subjective scales we review here include the Lysholm score, the Knee Injury and Osteoarthritis Score, the Knee Outcome Survey, the American Academy of Orthopaedic Surgeons Sports Knee Instrument, and the Quality of Life Measure. We also review the widely used comprehensive instruments, including the Cincinnati Knee Rating System, the International Knee Documentation Committee Knee Evaluation Form, and the Hospital for Special Surgery Knee Score. We present analysis of several patellofemoral instruments and activity scales, and present and discuss studies that have compared various knee outcome measures.