Arthroscopy of the knee joint is now fully established in the realm of diagnosis. Its techniques are standardized and its validity undisputed. It has replaced and rendered obsolete trial arthrotomy of the knee. In the future, diagnostic arthroscopy will have to compete with other imaging techniques such as computed tomography (CT) and magnetic resonance imaging (MRI). It is not yet possible to say whether ultrasonography will ever be able to provide noninvasive diagnosis of knee joint problems.