This paper sets out to provide a basis for a specification methodology for modelbased diagnostic systems (MBDS). The purpose of the methodology is to provide a mapping from the problem space of possible diagnostic applications to the solution space provided by the various approaches to MBDS. Therefore, given the major characteristics of a diagnostic problem, the methodology should provide guide-lines by which the specification of a suitable MBDS may be determined. As a first stage in the development of this methodology we provide taxonomies of the problem and solution spaces. The former is characterised via a set of Problem requirements, divided into Task, Fault and Model requirements, while the latter is classified as a set of System specifications, with reference to the major functional blocks of MBDS: namely the Diagnostic Strategist, the Predictor and the Candidate Proposer. The last part of this paper proposes a mapping between these two multi-dimensional spaces. This mapping is preliminary, and therefore neither exhaustive nor exclusive, but together with the taxonomies of the problem requirements and system specification will provide a catalyst for the development of a more extensive methodology. © 1994 J.C. Baltzer AG, Science Publishers.