Background: It is unclear how each bundle of the posterior cruciate ligament contributes to posterior knee stability. Hypothesis: Changes in bundle orientation and length occur such that neither bundle dominates in restraining posterior tibial motion throughout knee flexion and extension. Study Design: Controlled laboratory study. Methods: Six fresh-frozen cadaveric knees were studied in a joint-testing rig with individual quadriceps and hamstring muscle loading. Kinematic data for the tibia and femur were obtained at knee flexion angles from 0° to 120°. The joint was then disarticulated, and the insertions of the two bundles on the tibia and femur were digitized. Results: Length of the anterolateral bundle increased with increasing knee flexion angle from 10° to 120°. Length of the posteromedial bundle decreased with increasing knee flexion angle from 0° to 45° and increased slightly from 60° to 120°. Length of the anteromedial bundle was significantly less than that of the posteromedial at 0°, 10°, and 20° of knee flexion. The anterolateral bundle was significantly more horizontal at flexion angles of 0°, 10°, 20°, 30°, and 45° (P < 0.05). The posteromedial bundle was more horizontal at 120°. Conclusions: Changes in orientation take place such that neither bundle dominates in restraining posterior tibial motion throughout knee flexion and extension. Clinical Relevance: Double-bundle reconstructions achieve more physiologic knee function.