The tissue specific expression shown by intermediate filament proteins, their ease of extraction and their antigenicity has led to the use intermediate filament antibodies in diagnostic pathology, particularly antibodies to keratin intermediate filaments because of the predominant involvement of epithelia in cancers. We review the principles of differentiation-specific keratin expression and detail 100 of the best-known monoclonal antibodies to keratins which are currently in use: in many circumstances antibodies can now be reliably used as in situ markers of keratin expression. We also suggest a database scheme for better dissemination of information on available monoclonal antibodies to keratins in the future.