The biceps femoris is the most lateral component of the so-called hamstring muscles. Classically, this muscle's insertion into the head of the fibula has been described but further details of its anatomy have not been universally appreciated. Additional insertions into the crural fascia and tibia have been described. We dissected 56 cadavers paying especially close attention to the insertion of the biceps femoris muscle. The tendon of this muscle was found to have both medial and lateral slips each with an anterior and posterior component. Further, we found an attachment not only into the lateral condyle of the femur but also, the popliteus tendon and arcuate popliteal ligament. Our study has found that the tendon of insertion of the biceps femoris muscle is more complex than described previously and suggests that this tendon may be far more important in knee stability based on the multiple attachment sites found. We hypothesize that there may be a synergistic effect between the biceps femoris and the popliteus muscles based on our findings of an additional attachment of the biceps femoris tendon into the popliteus tendon. This study provides new detailed nomenclature for the description of the tendon of insertion of the biceps femoris muscle and indicates that the current description of the insertion of the tendon of the biceps femoris muscle should be revised. The clinician must have a thorough understanding of this anatomy before correct therapeutic maneuvers can be implemented. © 2005 Wiley-Liss, Inc.