The paper describes a development approach for knowledge-based systems (KBS) combining semiformal and formal techniques for specification and design. For the semiformal representation we use a hypermedia-based formalism which serves as a communication basis between expert and knowledge engineer during knowledge acquisition. The semiformal knowledge representation is also the basis for formalization, resulting in a formal and executable model of expertise specified in the Knowledge Acquisition and Representation Language (KARL). In the design phase, this representation is enriched by focussing on issues of realization and facilitating the mapping of the model of expertise to the implementation environment. A smooth transition from a semiformal to a formal specification and further on to design is achieved as all the description techniques rely on the same conceptual model to describe the system. Thus, the system is thoroughly documented at different description levels, each of which foc...