Autoimmune hepatitis (AIH) is a chronic and progressive liver disease, typically characterized by hypergammaglobulinemia, the presence of autoantibodies, and interface hepatitis. This chapter outlines the clinical presentation, laboratory findings, and diagnostic criteria for AIH, including a comparison between type 1 and type 2 AIH and various histological diagnostic criteria. While classic pathological features such as portal lymphoplasmacytic inflammation with interface hepatitis, emperipolesis, and hepatocyte rosettes are key indicators, other histologic findings like lobular hepatitis, perivenular/ centrilobular lymphoplasmacytic necroinflammation, Kupffer cell hyaline globules, and post-infantile giant cell hepatitis can also be variably seen in AIH. Additionally, this chapter will address the differentiation of AIH from drug-induced liver injury, and discuss the complexities involved in diagnosing AIH overlap syndromes.