The basic principles, technical objectives and future aspects of telecommunication in pathology (telepathology) are discussed. Visual information, which is the usually the basic source of medical disease classification, and its transfer have to be distinguished from acoustic information, which is commonly of secondary, i.e. abstract nature. Visual telecommunication in pathology can be applied in five different aspects, namely: a) to create visual data bases, b) to be used in expert consultation, c) to combine morphological and clinical data, d) to quantitate morphological findings (DNA- measurements, etc.), and e) to install remote control microscopes. Depending upon the application, different technical prerequisites and constraints have to be taken into account: queries of visual data bases and expert consultations can be performed by use of "normal" telephone lines; combination of morphological and clinical data and quantitation of morphological data may need a digitized telephone network (ISDN), and the transfer of live images needed for application of remote control microscopes can only be performed by use of ISDN or broadband networks (optical fiber transmission systems, satellite communication, etc.). The experiences of various groups working in different aspects of telepathology are described.