A series of experiments investigated the role of prior knowledge in tasks involving the operation of equipment from written instructions. The experiments covered two situations. In the first, the prior knowledge was already possessed by the subjects before the experiments. The studies involved comprehension and memory of technical prose, expertise in descriptions of familiar and unfamiliar pieces of equipment, and expertise effects in following instructions that differed in organization. In the second situation, the prior knowledge was provided as part of the training involved in the experiments. These studies concerned the role of knowledge of how a system works and transfer of training from previously learned operating procedures to new procedures. The results support methodological, theoretical and practical conclusions. Methodologically, many traditional prose recall paradigms should be used with caution in the investigation of prior knowledge, and careful attention should be paid to the relationship between the knowledge being supplied to the subject and the exact tasks that the subject is expected to perform. Theoretically, the results support what is perhaps becoming the consensus model of cognitive architecture, namely the ACT class of theories described in Anderson's most recent textbook, the "Architecture of Cognition" (1983). Practically, the results provide a good foundation for future applied research on the arrangement, sequence and content of instructional materials. (FL)