Aptitude-treatment interaction research is reviewed, with particular emphasis on the interaction of achievement and instructional method. Previous research has suggested that students with lower prior achievement require maximal instructional support. Recent research on this relationship is reviewed, as well as the relationship between intelligence and prior achievement. The distinction between aptitude-treatment interaction and achievement-treatment interaction is also examined. Five research studies of prior achievement and instructional method are reviewed. Three studies investigating the relationship between intelligence and prior achievement and the implications of pretesting are also discussed; a distinction is made between pre-task data and within-task variables. Researchers have recently been interested in the psychological processes underlying aptitude or intelligence testing and instruction. This concern is highly related to the achievemement-treatment formulation. Directions for further research is suggested. (GDC)