This document focuses on how research which investigates the interaction between learning abilities and instructional treatments (Aptitude Treatment Interaction or ATI) should proceed. Previous research related to ATI is evaluated in the context of the ATI premise that characteristics of learners affect their attainment of educational goals (outcomes from treatments). Previous research was found to be inadequate because of weak methodology, inappropriate hypotheses, and lack of replication. Guidelines for future research, introduced throughout the document, encompass design, methodology, and conceptual stages (such as understanding how general ability enters into a pupil's learning). In relation to two ATI goals (different instructional methods for different kinds of students should be employed to achieve the same educational goals, and personality dimensions as well as aptitude should be a criterion for placement rather than for rejection or selection in a program), research evaluation uncovered the following: learning rate is a false issue; general ability is related to learning in conceptual tasks; rote and meaningful instruction may serve different kinds of students; the principles governing the matching of learner to individualized instructional environment are not yet known; and the thinking on personality variables as they relate to instruction is in a primitive state. A 221-item reference list is included. (LP)