The ulnar collateral ligament (ucl) of the elbow also know as the medial collateral ligament is on the medial side of the elbow attaching to the medial epicondyle of the humerus and the sublime tubercle of the ulna. There are three parts of this ligament but the anterior bundle in the primary stabilizer against valgus, a force that happens during overhead throwing. Testing the ucl for valgus instability requires applying a valgus force in various positions and in various ways including long duration static testing, ballistic testing, and testing with and without distraction. The mcl when intact prevents gapping, distraction, and shear at the humeroulnar joint. The dynamic stabilizers of the elbow, predominantly Pronator Teres and the flexors, assist in resisting valgus stress and should be considered when assessing the integrity and stability of the joint. The biomechanics of the throw will be the determining factor, in most cases, for what steps will follow.