We examined insertion of patellar tendon and medial femoral collateral ligament in adult dogs by light and electron microscopy. Division of insertion sites into four zones—tendon, fibrocartilage, mineralized fibrocartilage, and bone—produces a model well suited for descriptive purposes. An almost parallel array of collagen fibrils makes this an ideal model for study of mineral collagen relationships. Preferential distribution of a 400-Angstrom dense mineral band across the C2, D, E1, E2, and A intraperiod bands corresponds well to the holes or non-bonded zones in collagen. Chondrocytes in mineralized fibrocartilage remain active despite partial enclosure by mineralized matrix. We have described anatomical features and relationships of collagen fibrils, elastic fibers, cells, ground substance, and mineral and discussed the physiologic significance of morphologic findings. We agree with Kolliker39 who, in 1853, wrote, "Close to the bones, the tendons frequently contain, throughout a certain extent, delicate, isolated cartilage cells, which are sometimes, however, contiguous and disposed in small rows. In exceptional cases, I have also seen the tendinous fibrils, at their extremities next the bone, entirely incrusted with calcarous salts, in the form of granules (ossified)."