Inflammation is characterized by the accumulation of leukocytes and other mesenchymal cells at sites of injury or infection. When inflammation results from invasion by a microorganism, the inflammatory response is designed to localize, destroy, and eliminate the offending organism. However, persistent inflammation in the absence of an identifiable irritant or microorganism is often as destructive to the host as any invader; it is, of course, the hallmark of such inflammatory diseases as rheumatoid arthritis. The recent explosion of information on the molecules which govern the emigration of leukocytes from blood vessels and their accumulation at loci of inflammation has completely changed our understanding of inflammation and may lead to new strategies in the treatment of acute and chronic inflammation. We describe here current ideas as to the pathogenesis of inflammation, the biochemistry and function of the adhesive molecules involved in inflammation, and what we know of their role in rheumatic disease. We also indicate how drugs presently used to treat these diseases affect adhesion molecules.