Approximately 18% of patients with auto-immune liver disease present with features characteristic of a second auto-immune liver disease. These cases have been termed "overlap syndromes."The pathogenesis of overlap syndromes is poorly understood, and few data are available regarding the clinical characteristics and outcome of this disease. Therefore, a consensus on the definition of overlap syndromes has not been reached. A common genetic background between auto-immune hepatitis (AIH) and its overlap with primary biliary cirrhosis(PBC) or primary sclerosing cholangitis (PSC) might confer susceptibility to a more inflammatory phenotype, probably requiring combined immunosuppressive treatment. This article focuses on the prevalence, diagnosis, and treatment of the overlap syndrome of AIH and PBC or PSC.