Mammals respond to an unexpected reward omission or reduction with a variety of behavioral and physiological responses consistent with an aversive emotion traditionally called frustrative nonreward. This review focuses on two aspects of frustrative nonreward, namely (1) the evidence for an aversive emotional state activated by the surprising omission or reduction of a rewarding outcome, and (2) the adaptive value of frustration. Frustrative nonreward has been mainly studied in terms of its mechanisms, across development in rats and across vertebrate species in comparative research. However, its adaptive function remains obscure. Following Domjan's approach to animal learning, this article explores a specific adaptive function hypothesis of frustrative nonreward called the incentive disengagement hypothesis. According to this hypothesis, the adaptive function of frustrative nonreward is to break an attachment to a site, situation, or stimulus that no longer yields appetitive resources (especially food and fluids) to promote the search for rewards in alternative locations. This function is of particular relevance given that mammals are especially vulnerable to reward loss due to their high metabolic rate and the energy demands of their relatively large brain.