The inflammatory diseases of the bones and joints encompass infections and the consequences of immunologically mediated local and systemic disease. Infections involve bones (osteomyelitis) and joints (septic arthritis) separately as well as together and result in necrosis with inflammatory features determined by the duration of the infection. In many cases, the infecting organism, whether bacterial, fungal or mycobacterial, is present within the infected site, but occasionally is no longer identifiable locally despite the persistence of infection-related phenomena. Granulomatous infections in bones and joints require distinction from Sarcoidosis. The diagnosis of the immunologically mediated inflammatory diseases, such as RA, depends as much on the clinical features as on the histologic ones, with a few findings that might point to one or the other in ambiguous cases. Any discussion of inflammatory arthropathies should at least mention Osteoarthritis, if for no other reason than to compare it with the traditionally regarded inflammatory diseases. However, there has been increasing interest on the potential role that synovial inflammation may play in the pathogenesis of this vary common arthritis. Ultimately, the diagnosis of the inflammatory diseases of the bones and joints requires the synthesis of information from many sources: clinical, serological, microbiological, radiographic, and pathological.