Hypermedia systems and knowledge systems can be viewed as #ip-sides of the same coin. The former are designed to convey information and the latter to solve problems; developments beyond the basic techniques of each system type requires techniques from the other type. In this paper, we introduce the concept of knowledge-based or intelligent hypermedia and analyse various constellations of merged hypermedia and knowledge systems. A hypermedia system deals with informal and formalized theories and the relations between and within these. Therefore, the corner stones of our analysis are the very basic notions involved in formalizing domain knowledge: an informal domain theory, a formal object theory axiomatizing the informal theory and a metatheory analysing the properties and interrelations between and within these. We integrate these notions into a system architecture which is to serve as a programmable system schema for supporting the composition of actual intelligent hypermedia systems. Programming in the large is supported by the schema which de"nes the overall system structure whereas programming in the small is supported by knowledge modelling techniques. The application of the system architecture is illustrated by the construction of an interactive diagnosis system which involves knowledge-based reasoning both for navigation in hyperspace and problem-solving within the domain.