Health systems turn everywhere, but with different speed, from organization-centered to personalized eHealth or pHealth, i.e. ubiquitous care delivery independent of time and location of the resources involved. As interoperability is an important issue in such distributed, fully integrated, intelligent and individualized environment, pHealth solutions have to comply with advanced architectural solutions based on international standards. Representing concepts and their interrelations, such architectural framework perspectives' system architecture, domains, and development process can be described by the domains' ontologies. Therefore, advanced interoperability approaches have to refer to ontology principles, finally resulting in ontology-driven approaches to semantically interoperable and sustainable health information systems. The paper investigates functional requirements, interoperability levels, architectural approaches to pHealth systems, thereby analyzing and classifying related existing or emerging standards.