A great deal of experimental work has conducted since Falk's report of excessive water consumption in animals exposed to periodic schedules of food delivery. Before reviewing the experimental work on opponent-process theory, this chapter briefly outlines four major reasons why we view the theory as relevant to adjunctive behavior. The first reason is that periodic delivery of food to hungry animals induces a wide variety of behaviors which depend on the species used and the environmental stimuli available in the experimental context. The second reason is that schedule-induced behaviors, such as polydipsia and attack, begin shortly after pellet consumption, quickly rise to a maximal level, and gradually decrease during the interpellet interval. The third reason is that experimental manipulations, which should theoretically affect the intensity of the A-, and consequently the B-process, are known to regulate the strength of adjunctive behaviors. The fourth reason is that if the interpellet interval is sufficiently long, polydipsia is not observed.