Posterolateral injury is often associated with additional ligamentous injury to the knee. Proper evaluation of these injuries requires a knowledge of the anatomy and biomechanics of the posterolateral corner. Biomechanical studies reveal that the popliteal tendon, the popliteofibular ligament, the arcuate ligament, and the LCL are the principal structures responsible for maintaining stability of the posterolateral corner. Knowledge of the biomechanics of the posterolateral corner provide insight into the clinical evaluation and treatment of posterolateral knee instability. Patients with acute and chronic posterolateral instability present with complaints of knee pain and instability of the knee in extension. Patients may also complain of dysthesias or motor weakness owing to associated peroneal nerve injury. Patients may also present with symptoms pertaining to associated ligamentous injury to the knee. Thorough examination is needed to document neurovascular status, and the presence of posterolateral and associated knee instability. Although many tests are used to document posterolateral knee instability, the authors prefer varus stress testing and the prone external rotation test. Accurate diagnosis of all knee ligament pathology will allow appropriate planning for the surgical treatment of isolated posterolateral knee instability and combined instability patterns.