Autoimmune hepatitis (AIH) has been defined as "an unresolving, predominantly periportal hepatitis, usually with hypergammaglobulinemia and tissue autoantibodies, which is responsive to immunosuppressive therapy in most cases." It is a relatively rare disorder, with a preponderance of female patients, that can present at any age (although onset in most cases is after 40 years of age). There are no features that are pathognomonic of the condition. Diagnosis requires careful exclusion of other causes of liver disease together with the finding of a suggestive pattern of clinical and laboratory abnormalities. The marked heterogeneity of AIH with respect to presenting features, severity of disease, and response to therapy has led to several proposals for classification of the disease according to (mainly) immunologic parameters. These schemata may assist in diagnosis, and some may define different pathogenetic subgroups of the disease, but their utility for assessing prognosis or planning treatment strategies for the individual patient is still uncertain.