The purposes of the present study were to resolve a number of major inconsistencies found in the literature on the structure of the quadriceps femoris muscle and to extend knowledge of its structure using descriptive, qualitative methodology. The quadriceps femoris muscle was investigated in 41 cats, and the findings were confirmed in 6 human cadavers. Two aponeuroses with major biomechanical functions (rectus-vastus and vastus aponeurosis), neither of which had been previously described in the literature, were characterized in both species. The study also resolved many major inconsistencies in the literature: The muscle sometimes described as vastus intermedius (VI) was found to be the articularis genu, the muscle sometimes described as vastus medialis (VM) was found to be the VI, the rectus femoris head was found to have an additional proximal nerve branch not previously recognized, no anomalous 5th head was ever found, and the distal VM were not found to have 2 heads (in either cats or humans). The authors’ anatomical descriptions and bimechanical models of the muscles, tendons, and neurovascular should provide a helpful foundation for future studies on the quadriceps. Two general recommendations are made: 1) that the feline model be considered a viable model to elucidate human knee pathomechanics; and 2) that regardless of the anatomical structure of interest, orthopedic nurses, orthopedic surgeons, and research investigators should routinely use the research literature for anatomical guidance instead of standard anatomical textbooks. © 2002, Sage Publications. All rights reserved.