In the context of a research project on the role of prior knowledge state (PKS) in the learning process in a modular education system, a study of the literature was done in order to construct a conceptual map of "prior knowledge." In addition, a study was carried out in order to operationalize the concept of prior knowledge, to construct a conceptual map of prior knowledge terminology, and to investigate how the PKS could be operationalized using a number of variables. A questionnaire was developed to examine conceptualizations of the PKS and methods of establishing it. The questionnaire was sent to 27 Dutch and Flemish speaking experts who research prior knowledge, and responses were received from 17. Also explored were methods for measuring a student's PKS. Data collected from the literature review and from an enquiry among specialists are reproduced and interpreted to form a proposed model for research into PKS. Fourteen figures illustrate the discussion, with figure 5 depicting the proposed model and figures 1 and 2 in Dutch. (Contains 239 references.) (SLD)