A model of the human knee is described in which the cruciate, collateral and capsular ligaments are represented by thirteen elements. The coordinates of the attachment sites and the dimensions of the ligaments are made by in vivo and in vitro measurements. The stiffness of the model is then assessed at flexion angles between 0° and 90°, under varus load, valgus load, internal and external rotation and anterior and posterior displacement. The theoretical results are then compared to experimental results and good agreement is achieved. The relative effect of a given ligament is then investigated by eliminating the element in the model and comparing this to an actual test where the ligament is transected. The importance of the deep fibers of the medial collateral in the anterior-posterior, rotational and valgus directions is demonstrated. The function of the oblique fibers of the medial collateral is shown as is the rotational stability derived from the anterior cruciate ligament.