Mycoplasma hyopneumoniae is the primary agent of enzootic pneumonia in pigs. The lung lesions, generally observed in young pigs, are characterised by a hyperplasia of the epithelial cells and an increased perivascular and peribronchiolar accumulation of mononuclear cells. Following M. hyopneumoniae infection, immune reactions are observed and resistance is induced in pigs. Laboratory diagnosis is generally performed by an immunofluorescent test and by enzyme-linked immunosorbent assay. Antibiotics are useful but the development of resistance has been described. Vaccination seems to be an effective method of controlling the disease. M. hyorhinis, generally transmitted by sows to piglets through nasal secretions, exists in a high percentage in the respiratory tract of healthy pigs. But some strains can induce serofibrinous to fibrinopurulent polyserositis and arthritis. M. hyorhinis is isolated from acute and subacute phase lesions and serum antibodies are detectable. M. hyosynoviae has a special affinity for joint tissue and may cause arthritic disease, leading to economic losses. This mycoplasma is generally located in the tonsils. Piglets are infected by sows after four to six weeks of life. Evidence of disease occurs in animals of between 30 to 40 kg and 100 kg, and bursae and joints are affected. A non-suppurative viscous fluid of a serofibrinous/serosanguineous nature is reported. In chronic cases, the synovial membrane is affected. M. hyosynoviae is isolated from the joints and pharyngeal/tonsillar samples and can induce antibodies in blood and joint fluid. Predisposing factors play an important role. M. flocculare is widely distributed in swine, in normal and pneumonic lungs and in nasal cavities, but no pathogenic capability has been described. There is great interest in this mycoplasma because of the great similarity to M. hyopneumoniae.