Autoimmune disease occurs when the immune system attacks self-molecules as a result of a breakdown of immunologic tolerance to autoreactive immune cells. Many autoimmune disorders have been strongly associated with genetic, infectious, and/or environmental predisposing factors. Comprising multiple disorders and symptoms ranging from organ-specific to systemic, autoimmune diseases include insulin-dependent diabetes mellitus, rheumatoid arthritis, systemic lupus erythematosus, scleroderma, thyroiditis, and multiple sclerosis. There are also implications of autoimmune pathology in such common health problems as arteriosclerosis, inflammatory bowel disease, schizophrenia, and certain types of infertility. Largely of unknown etiology, autoimmune disorders affect approximately 3% of the North American and European populations, > 75% of those affected being women. This discussion provides a brief introduction to the immune system and tolerance maintenance, an overview of selected autoimmune diseases and possible mechanisms of immune autoreactivity, and a review of experimental autoimmune models.