Diseases of synovial joints can conveniently be divided into degenerative arthritis and inflammatory arthritis on the basis of the predominant pathologic process (Table 20). Degenerative arthropathies are the most common types and include traumatic arthritis and osteoarthritis. Inflammatory arthropathies are less common than degenerative arthropathies and have either an infective or immune-mediated etiology. Infective arthritis caused by bacterial infection (septic arthritis) is the commonest type of inflammatory arthritis in the cat. Septic arthritis is classed as an erosive type of arthritis because there is destruction of articular cartilage in joints infected by bacteria. Immune-mediated arthropathies can be subdivided into both erosive and nonerosive forms. Differentiation between the infective and immune-mediated forms of inflammatory arthritis is essential since the therapeutic approaches to these conditions are diametrically opposed. Inappropriate treatment of an infective arthritis with immunosuppressive drugs will have disastrous results.