Stimuli that precede aversive events are typically less preferred than stimuli that precede nonaversive events. It has recently been demonstrated that stimuli that "follow" less preferred events may become favored more than stimuli that follow more preferred events. This phenomenon has been investigated under a variety of names, most commonly, "within-trial contrast" and "state-dependent valuation". Although this effect has been replicated, there have been several failures to replicate and it is still little understood. This paper reviews and summarizes the literature on within-trial contrast and state-dependent valuation. Procedural variations across studies are identified and discussed. The two current models that explain the phenomenon are then outlined and the limitations of each model are described. A third explanation is offered that incorporates the concept of motivating operations. Last, the predictions of all three models are compared. (Contains 2 figures and 1 footnote.)