The overhead throwing motion subjects the elbow to a predictable pattern of forces, including medial tension, lateral compression, and posterior shear, that in turn result in a predictable pattern of injuries. Careful history taking, thorough physical examination, and judicious diagnostic imaging allow clinicians to correctly diagnose ulnar collateral ligament (UCL) injury. Athletes with UCL injury complain of acute or chronic medial elbow injury, resulting in decreased throwing effectiveness, with loss of control and/or velocity. Magnetic resonance imaging is the gold standard for diagnosis, but stress ultrasound rapidly is becoming an important adjunct, particularly in diagnostically challenging situations.