Intrahepatic bile ducts (IHBDs) develop from bipotential liver progenitor cells in contact with the mesenchyme of the portal vein and thus form the "ductal plates." The ductal plates are remodeled into mature tubular ducts. Lack of remodeling results in the persistence of periportal epithelial sleeves or "ductal plate malformation" (DPM). A proposal is that virtually all congenital diseases of IHBDs represent examples of DPM. Some early, severe types of extrahepatic bile duct atresia are characterized by DPM, a suggestion of a prenatal beginning of the disease. Several congenital diseases are characterized by dilatation of segments of IHBDs and variable degrees of fibrosis. Such "fibrocystic diseases" represent DPM at different levels of the biliary tree. Autosomal recessive polycystic kidney disease represents DPM of interlobular bile ducts, associated with tubular dilatation of collecting renal tubules. Congenital hepatic fibrosis may derive from the same type of liver lesion, through a superimposed destructive type of cholangiopathy associated with scarring fibrosis. Caroli's disease represents DPM of the larger IHBDs, whereas Caroli's syndrome combines the lesions of Caroli's disease and congenital hepatic fibrosis. von Meyenburg complexes represent DPM of smaller interlobular ducts; their dilatation gives rise to the liver cysts in autosomal dominant polycystic kidney disease. Finally, DPM is a component of the tissue abnormalities in so-called mesenchymal hamartoma.