Medial knee instability is a serious knee injury that is common with cruciate ligament tears and multiple ligament knee injuries. There are varying degrees of medial instability with respect to pathologic axial rotation and valgus laxity. Treatment of medial instability must address all components of the medial instability pattern which include the medial and posteromedial capsule, the posterior oblique ligament, the superficial medial collateral ligament (MCL), and the semimembranosus insertion sites, as well as other structural knee injuries that are present. The successful treatment of posterior cruciate ligament and anterior cruciate ligament (ACL) injuries depends upon recognition and treatment of the associated medial side injuries. This issue of Sports Medicine and Arthroscopy Review is dedicated to the evaluation and treatment of medial instability of the knee. It has been an honor to work with this distinguished group of contributing authors who are experts in the evaluation and treatment of medial instability of the knee. Authors LaPrade, Kennedy, Wijdicks, and LaPrade present the anatomy and biomechanics of the medial side of the knee, and their importance to surgical reconstruction. The authors discuss that in order to reconstruct the medial knee to restore the original biomechanical function of its ligamentous structures, a thorough understanding of its anatomic placement and relationship with surrounding structures is required. To restore the knee to normal kinematics, the diagnosis and surgical approach have to be aligned to successfully reconstruct the area of injury. Three important ligaments maintain primary medial knee stability: the superficial medial collateral ligament, the posterior oblique ligament, and the deep medial collateral ligament. It is important not to exclude the assistance that other ligaments of the medial knee provide, including support of patellar stability by the medial patellofemoral ligament and multiligamentous hamstring tendon attachments. Valgus gapping and medial knee stability is accounted for collectively by every primary medial knee stabilizing structure. Drs Kurzweil and Craft discuss the physical examination and imaging studies for evaluating medial side knee injuries. The authors emphasize that a detailed physical examination can help determine the severity of the medial-sided injury. When combined with advanced imaging, the examination will delineate damage to associated medial knee structures, including the location of MCL damage, posteromedial capsule injuries, and combined cruciate injuries. Failure to recognize MCL injuries that may be prone to chronic laxity can lead to significant disability, joint damage, and failure of concomitant cruciate ligament reconstructions. The authors discuss the importance of magnetic resonance imaging, stress radiography, and ultrasound combined with physical examination for the evaluation of medial knee injuries. Drs Taylor and Roth address the topic of management of acute isolated medial posteromedial instability of the knee. Taylor and Roth present the idea that medial-sided knee injuries are very common, and that the medial collateral ligament is the most commonly injured ligament of the knee. Injuries to the medial side of the knee can occur in isolation or concomitant with other knee ligament injuries. Isolated grade I and II injuries have been typically treated nonoperatively with excellent results. Isolated grade III injuries, however, are less common and more controversial. Although some recent literature has shown acceptable results with nonoperative treatment of isolated grade III injuries, most authors recommend surgical treatment. The authors emphasize that a variety of surgical techniques have been described, all with favorable outcomes. The surgeons’ choice of treatment method should be based on injury pattern with the goal of regaining valgus and anteromedial rotator stability of the knee. Drs Wascher, Menzer, and Treme address surgical treatment of medial instability of the knee. The authors discuss several MCL reconstruction options, and outline the authors’ preferred MCL reconstruction surgical technique.