This report collects together three presentations given by the author at scientific and professional meetings during the Spring of 1977. The first discusses present views of the variety and complexity of individual differences among human beings in the context of current research on cognitive processing in learning and instruction. The second section presents an eight-point outline of the nature of prescriptive instructional theory and shows how considerations of individual differences are involved at each point. The third section reviews how data on aptitude-treatment interactions can be collected and used by the instructional designer in the instructional development and revision process. (Author)