A comprehensive literature review of the published articles which have tested, evaluated, or modified the conditioning theory specified by Rescorla and Wagner (1972) is presented. Following a description and presentation of the formal properties of the model, five basic classes of studies are reviewed. The classes are comprised of reviews of studies which have (a) tested the model’s basic assumptions, (b) manipulated the salience of conditioned stimuli, (c) manipulated the potency of unconditioned stimuli, (d) investigated background stimuli or commom»elements between stimuli, and (e) introduced refinements or modifications of the original model. Finally, a brief discussion of the theory’s impact and future is presented.