Adults with arthritic conditions are seen frequently in primary care clinics. However, more than 100 different entities can produce joint and muscle symptoms, which makes it challenging to correctly diagnose musculoskeletal complaints. There are several logical steps to follow in assessing joint disorders. The first is to differentiate between what is and what is not arthritis. Additional steps necessary for an appropriate diagnosis include analysis of a thorough history, physical examination, and laboratory and X-ray results. It is of critical importance to identify the most common forms of arthritis, as well as the specific conditions that require immediate referral. The onset, incidence, findings and pathophysiology of the following entities in the adult population are discussed: septic arthritis, osteoarthritis, rheumatoid arthritis, the crystal-induced diseases, human immunodeficiency virus (HIV) and arthritis, the seronegative spondyloarthropathies and systemic lupus erythematosus.