The medial collateral ligament complex is a primary stabilizer that combines static and dynamic resistance to direct valgus stress while contributing significant restraints to rotatory motion and anterior-posterior translation. Varying opinions exist among investigators regarding injury classification and treatment algorithms. Whereas most agree that the majority of isolated medial collateral ligament complex injuries can be treated nonoperatively, isolated injuries with chronic instability and multiligament injuries may require operative intervention. Substantial confounding factors are present within published reports, making comparative analyses and systematic review challenging. This review focuses on the anatomy and biomechanics of the medial structures of the knee; it discusses the clinical evaluation of complex injuries; and it reviews nonoperative and operative treatment methods.