In the last decade, research on computer-based instructional systems has been heavily influenced by rapid advances in cognitive science and artificial intelligence, and by remarkable increases in the power of computers. Due to these factors, there has been a shift from traditional computer-based instructional systems to knowledge-bused systems in which knowledge representation techniques are used to model the domain to be taught, the thinking and learning processes of the student, and the applied strategies and tactics for instruction and presentation. Knowledge-based instructional systems may take the form of intelligent tutoring systems (ITSs), adaptive help systems, microworlds, simulations with student-sensitive instructional support, and so forth. The knowledge bases of those systems must be suitable for interpretation by inference engines in order to select appropriate pieces of knowledge for making diagnoses, predictions, and evaluations that steer the instructional process. Amongst many others, two prevailing problems seem to emerge in this field of research. First, large bodies of knowledge that are required for the development of knowledge-based instructional systems are not available in a useful format. For instance, both theories of learning and theories of instruction often display a vagueness and implicitness that obstructs their implementation in knowledge-based systems. What is needed is experimental research that is aimed at the explication of this knowledge. Second, the extreme interrelatedness of the knowledge that must be present in instructional systems places heavy demands on current knowledge representation, storage, and inference techniques. Obviously, more effective conceptual models relating to the architecture and design methodology of knowledgebased instructional systems are needed in order to properly handle their complexity. This special issue provides an overview of Dutch research on knowledge-based instructional systems. As will be shown in the next section, this research pertains both to experimental research, aimed at the formulation and explication of knowl-