This study aimed to provide an anatomical description of the anterolateral ligament (ALL) of the knee and characterize its biomechanics. The ALL was found in all 35 cadaver knees and was an important internal rotatory stabilizer, especially between 30-60 degrees of flexion. Cutting the ALL increased the pivot shift grade in the ACL-intact and deficient knee. The ALL was identified on 95.3% of MRI scans of ACL-injured patients, with lesions in 79.2%. The Segond fracture was delineated as a bony avulsion of the ALL.