Arthroscopy for degenerative conditions of the knee is among the most widely employed orthopaedic procedures, but its effectiveness (like the effectiveness of many surgical operations) has never been proven in prospective trials. The evidence supporting the use of arthroscopy in treatment algorithms for knee OA comes largely from case series and cohort studies. These studies have shown that about 50% of patients report pain relief following the procedure. Predictors of poor outcome following arthroscopy include marked mal-alignment, restricted range of motion, advanced radiographic changes, and prior surgery. Improved outcomes are predicted by preoperative mechanical symptoms caused by loose bodies, meniscal tears, or radiographic evidence of mild articular degeneration.