Arthroscopy still remains the 'gold standard' for the assessment of articular cartilage and synovium because it provides direct and magnified evaluation of these anatomical structures. Thus, alongside the use of arthroscopy as a diagnostic or therapeutic procedure in knee disorders, a further function of knee arthroscopy, performed under local anaesthesia on an outpatient basis, has been proposed: the monitoring and follow-up of knee chondropathy and synovitis conducted for research purposes on patients suffering from knee osteoarthritis or chronic synovitis. This function is used in order (1) to evaluate the natural history of these diseases, (2) to assess 'de visu' the effect on chondral or synovial lesions of medical treatments or surgical interventions, and (3) to validate non-invasive imaging techniques such as plain radiographs or magnetic resonance imaging. The development of this arthroscopic outcome measurement of chondropathy and synovitis required the establishment and validation of systems for scoring the severity of chondral and synovial lesions. The author reviews both earlier and newer arthroscopic classifications and underlines the need for a quantitative description of cartilage and synovial abnormalities, either global and based on the investigator's overall assessment by using a visual analogue scale of severity, or, more analytically, taking into account the baseline parameters of the lesions, i.e. depth/intensity, extent and location.