Adjuvant arthritis (AA) is an induced form of (sub)chronic arthritis. Strains of rats have a varying genetic susceptibility to AA, whereas mice generally are not susceptible. AA is most easily induced with mycobacteria suspended in oil, although in some strains of rats it can be induced with oily adjuvants in the absence of mycobacteria. The disease is a T cell-mediated autoimmune arthritis that is frequently used to study immunological aspects of rheumatoid arthritis (RA) and other arthritic or inflammatory diseases in humans. Furthermore, it is used as a model for developing and testing antiinflammatory drugs. There are no particularly well-defined autoantigens in AA; in this respect, the model resembles spontaneous arthritic diseases in humans. In all susceptible rat strains, the inflammatory process of AA is self remitting, although usually the disease is severe and leads to permanent joint malformations, including ankylosis; a time line for AA development is presented. This unit describes the induction and evaluation of AA and the preparation of adjuvant used to induce AA.