Autoimmune Addison's disease is caused by autoreactivity towards the adrenal cortex involving 21-hydroxylase autoantibodies and autoreactive T cells. Autoimmune destruction of the adrenal cortex is triggered by hitherto unknown environmental factors in individuals with genetic susceptibility. Several genes have been identified, of which the major histocompatibility complex haplotypes DR3-DQ2 and DR4-DQ8 are most strongly associated. In addition, other genes also implicated in other autoimmune diseases are linked to Addison's disease, such as cytotoxic T lymphocyte antigen 4 (CTLA-4), protein tyrosine phosphatase non-receptor type 22 (PTPN22), major histocompatibility complex class II transactivator (CIITA), and most recently the C-lectin type gene (CLEC16A). Studies employing T cells in humans and animal models, and the collection of large patient cohorts facilitating genome-wide screening projects, will hopefully improve the understanding of the pathogenesis of the disease in the near future.