Autoimmune pancreatitis (AIP) is distinct from calcifying and obstructive forms of chronic pancreatitis. Clinically and histologically it has two distinct subsets: a) lymphoplasmacytic sclerosing pancreatitis or Type 1 AIP which appears to be a systemic disease characterized by abundant infiltration of affected organs with IgG4 positive plasma cells and b) duct centric or Type 2 AIP characterized by granulocyte epithelial lesions in the pancreas without systemic involvement. In AIP a marked lymphoplasmacytic infiltrate that responds dramatically to steroid therapy suggests an autoimmune etiology. However, the target autoantigen(s) and the effector cells in AIP remain speculative. Though elevated serum levels of IgG4 and a tissue infiltration with IgG4 positive plasma cells are consistently seen in Type 1 AIP, the role of IgG4 in its pathogenesis remains unknown. Recent development of animal models of AIP will help improve our understanding of the pathogenesis of these newly described forms of chronic pancreatitis.