Many clinical strategies use patients' imagery to explore and treat phobic and posttrauma reactions, however little attention has been paid to the underlying assumption that imagery of relevant stimuli may help maintain conditioned behavior. In this article, the authors examine the premise that mental images can potentiate and substitute for physical stimuli in human classical conditioning. The authors review empirical evidence to detail the role of images of conditioned stimuli (CS) and unconditioned stimuli (US) during pre-exposure to stimuli, the actual pairing of the CS and US, and extinction when the CS is presented alone. The evidence suggests that mental imagery can facilitate or diminish the outcome of classical conditioning in humans and, more tentatively, that mental images can substitute for actual US and CS in autonomic conditioning. They argue that researchers should explore the role of mental imagery in conditioning through the use of advances in the measurement of imagery. Finally, they analyze anxiety and trauma reactions as examples of how applied areas can be used to explore and benefit from developments in this area.