Motion and forces at the knee have clinical implications in reconstructive surgery. The discussion of these implications is based on the following biomechanical information. Normal knee motion during simple activities of daily living utilizes up to 110° of flexion-extension and 15° of abduction-adduction and rotation. The instant centers of motion and surface velocities become abnormal with intra-articular derangement. The average area of articular surface contact between the tibia and femur on the medial plateau is 1.6 times larger than the lateral. Under static conditions, the compression force per unit area is increased by deformity. The forces acting through the knee during walking approximate three times body weight. Rheumatoid and degenerative arthritis are associated with decreased stance phase flexion-extension.