The MCL restrains excessive valgus movement and external rotation forces on the medial aspect of the knee. It is the most commonly injured knee ligament. The usual mechanism of injury is a direct or indirect force on the lateral knee when the foot is planted or fixed. Acute MCL injury is associated with knee pain, effusion, joint instability, altered gait, and functional deficits. The severity of MCL sprains is graded on a 3-point (I, II, or III) scale (i.e., mild, moderate, and severe, respectively). Grade I refers to no valgus instability (microscopic tears only), whereas grade II represents valgus instability in knee flexion representing a partial tear of the MCL. In the most severe case (grade III), a complete MCL tear has occurred and valgus instability is evident in both knee flexion and extension. Other non-contractile structures, such as the cruciate ligaments or menisci might also be damaged in severe MCL sprain. Treatment depends on the severity of the injury, with surgery optional for cases with grade III tear. The prognosis for recovery in grade I or II MCL sprain is generally good to excellent. Individuals most at risk for MCL sprain are those who participate in contact sports, such as soccer, football,wrestling, hockey, judo, and rugby. A structured warm-up program might help to prevent knee and ankle injuries in young athletes at risk for MCL sprain.