In this paper the technique of knee arthroscopy is given in detail and thirty cases are summarized to indicate the value of the procedure. Arthroscopy can be done without fear of infection or of trauma to the joint. It has the advantages of a diagnostic arthrotomy without necessitating operation, and it is, therefore, applicable to many cases in which operation is either not permitted or inadvisable. It has, however, difficulties, some inherent in the method, others in the technique, which necessitate the most thorough cadaver practice before arthroscopy should be attempted on a patient. The authors are keenly aware of the present imperfections of arthroscopy, and one purpose of this presentation is to elicit the active help of their colleagues in perfecting this new diagnostic method.