Many knee injuries that previously caused disability can now be diagnosed accurately and treated successfully. Arthroscopy has been instrumental in this progress. The anterior cruciate ligament, which is an important stabilizer of the knee, is commonly injured. Disruption of the restraint provided by the ligament results in an unstable knee that can give way during pivoting motions. Early reconstruction is usually effective in restoring stability and improving function. Considerable progress has been made in knee bracing. However, prophylactic bracing has not been demonstrated to be effective in preventing knee injuries.