The authors describe the "Synovial Plica Syndrome", one of the lesser known and therefore often overlooked painful conditions of the knee. It is due to the persistence of remnants of the embryonic synovial septa which are normally innocuous but in certain circumstances (usually of mechanical origin) can undergo inflammatory changes resulting in fibrosis. This produces a clinical picture with characteristic and distinctive features. The anatomical and pathogenic aspects are described and the differential diagnosis and possible lines of treatment are discussed and illustrated. If the diagnosis is made early, the results of treatment are excellent.