A vast literature in educational psychology attests to the fact that individual differences in learner aptitudes predict learning outcomes. A substantial body of literature also demonstrates that aptitude variables often interact with instructional treatment variables in these predictions. Aptitude main effects and aptitude-instructional treatment interactions (ATI), frequently account for a larger proportion of variation in learning outcome than do treatment main effects alone. ATI findings, in particular, have important implications for the development of instructional theory and research and for instructional improvement. They suggest how instruction can be made adaptive to student differences. But if practical and theoretical use is to be made of these ideas, then individual differences in aptitude for learning need to be understood, at a more analytic level, as individual differences in psychological processes. Traditional research on aptitude sought mainly to improve the predictive power of measures, and to build a taxonomy of aptitude constructs based on correlational and factor analyses. With the growth of a cognitive experimental psychology of information processing, coupled with the development of ATI research on instruction, it now seems possible to pursue coordinated process analysis of aptitude and learning. This paper reviews some of the current research in this area, concentrating particularly on recent and ongoing studies conducted within the Aptitude Research Project at Stanford University. First, two ATI findings are described and then some strategies for research aimed at identifying aptitude processes are outlined. There seem to be five different approaches that have been used to measure aptitudes, and these are described briefly. (Author/AM)