As the arthroscopic surgery of the knee joint has taken rapid strides recently, the knowledge of the structure and deveIopment of the joint are getting needed more and more. An embryological study about the development of the human knee joint was carried out using a total of 23 knees of human embryos. Serial sections of 23 embryos aged 5-8 postovulatory weeks (12-31 mm C.R.) and 3-dimensional reconstruction were examined. The embryos have been staged according to Carnegie's 'developmental stages'. The femur, tibia and fibula had begun to undergo chondrification by stage 18. The patella had commenced chondrification at stages 23. As the mesenchymal model began to chondrify, concomitant changes occurred in the region of the presumptive knee joint to create the interzone (stage 19-20). The following structures became condensed successively: patellar tendon (18-19), lateral collateral ligament and popliteus tendon (19-20), cruciate ligaments (20-22), meniscus (21-22). In summary, the differentiation from a generalized cellular blastema to a joint resembling the adult in form and arrangement occurred in only a relatively few days. By the end of the embryonic period proper (stage 23, 8 postovulatory weeks), all the elements of the knee joint were present in a form and arrangement closely resembling those of the adult. It seems that the embryological studies are more required to clarify the structures having many variants such as synovial plica, meniscus, and the development of the joint cavity through this study.