The diagnosis of OA is suspected on clinical features but most often based on radiographic data. Radiographic criteria were proposed by Kellgren and Lawrence in 1957 and adopted by the World Health Organization in 1961. Lequesne proposed a radiological classification of OA excluding the spine in 1982. Most of the work on the classification of OA of the knees was achieved by Altman and the members of the American Rheumatism Association Subcommittee on Classification of OA in 1986. The presence of osteophytes seemed to best differentiate OA. The classification of OA of the knee can be made with 83% sensitivity and 93% specificity if the patient has knee pain and the radiographs demonstrate osteophytes. The choice of osteophytes, not cartilage damage, has been criticized. Currently the proposed OA criteria are the only organized attempt to define variables that separate OA of the knee from other conditions.