A logical definition of the anatomy of the tibial surface, based on the shape of the tibial surface as measured in the laboratory, is presented. The relevance of these data to a better understanding of normal and pathological knee joint function is discussed. The thesis that the spine of the tibia is the major weight-bearing portion of the tibial surface can be supported by this contour study. The significance of the lateral plateau contour relative to the static stability of the joint contributed by joint apposition has been indicated. In addition, the path followed by the lateral femoral condyle during the "screw home mechanism" and caused by joint contour has been clearly demonstrated. We hope that this experimental evidence will establish itself as the basis for understanding the anatomy of the proximal tibial surface and that the nomenclature can be accepted to provide a common base for discussion of the tibial surface.