A review is presented of the different aspects of heterogeneity that need to be resolved to achieve global interoperability, and specifically, to provide global transport service. It concentrates on the problems of interconnecting computer networks and systems that use different protocols, either different subsets of OSI (Open Systems Interconnection) standards or OSI and non-OSI protocols. The issues of protocol conversion are discussed, and integrated naming, addressing, and directory mechanisms for interconnecting OSI and non-OSI domains are outlined. More subtle aspects of interoperability are examined for the case of the OSI transport protocol class 4 in mixed LAN/WAN (local-area-network/wide-area network) environments.