Autoimmune hepatitis (AIH) is a progressive inflammatory liver disease with a female preponderance, responsive to immunosuppressive treatment. Two types of AIH are described: type 1 AIH is characterized by positivity for smooth muscle and/or antinuclear antibody, while type 2 AIH is positive for liver kidney microsomal type 1 antibody. The putative mechanisms leading to the development of this condition include genetic predisposition to autoimmunity through possession of specific human leukocyte antigen alleles, immune reactions to liver cell antigens, possibly triggered by a mechanism of molecular mimicry, where immune responses to external pathogens, e.g. viruses, become directed toward structurally similar self-components, and an impairment in immune regulation. AIH has been described to arise de novo after liver transplantation. The mechanisms leading to post-transplant autoimmunity remain to be defined.