This chapter describes anteromedial rotatory instability (AMRI) of the knee joint. AMRI results from excessive valgus strain with simultaneous external rotation of the knee, leading to pathologic anterior subluxation of the medial tibial plateau relative to the medial femoral condyle. The anatomical and biomechanical background for AMRI is presented and related to clinical evaluation, and treatment decision strategies including surgical techniques and clinical outcomes are discussed.