The epithelial layer covering the biliary system. This includes the epithelium of the gallbladder (when present) as well as the intrahepatic and extrahepatic bile ducts. [ http://purl.obolibrary.org/obo/BTO_0001513 http://www.informatics.jax.org/accession/cs http://orcid.org/0000-0002-6601-2165 ]