Educational psychology has indicated that prior knowledge is a potentially important educational variable. Recent and earlier research in educational psychology has shown that 30 to 60% of the variance in study results could be explained by this variable. Insight into these factors should influence the return on education. A common theoretical frame regarding explanatory theories and concepts, considering different sorts of prior knowledge and individual differences in prior knowledge states, could lead educational psychology research to previously acquired knowledge and experience. Further research should concentrate on such a framework, building on earlier results. This report gives an overview of research on the effect of prior knowledge on learning and theories that offer an explanation for the facilitating effect of prior knowledge. An exploration of the concept "prior knowledge" is made, largely on the basis of the English language literature. Attention is also paid to the expert-novice paradigm. Four figures and one table illustrate the discussion. (Contains 116 references.) (SLD)