The medial collateral ligament (MCL) is one of the four major ligaments of the knee. It is a broad, flat, membranous band, situated slightly posterior (back) on the medial (inner) side of the knee joint. It resists forces that would push the knee medially, which would otherwise produce valgus deformity, commonly referred to as "knock-knee." MCL tears often occur from soccer, skiing, or football and involve the joint being bent to the side, tearing the ligament that exists just inside the soft tissue of the knee. The MCL usually tears partially and is often graded as a Grade 1, 2 or 3 type of tear. Fortunately, the MCL has a very good blood supply. By protecting and rehabilitating it early with gentle range-of-motion exercises, soft tissue massage, and specific strengthening exercises, the tissue can be induced to heal in a relatively normal pattern with collagen fibers aligned along the normal pathway of the original MCL. Surgical repair of the MCL, in our opinion, is infrequently needed because the MCL will often heal. Occasionally, MCL injuries lead to chronic instability and in those cases we rebuild the MCL typically using an allograft or donor tissue to augment the suture repair of the ligament itself.