Foraging theory is a branch of behavioral ecology that deals with how animals seeking nourishment (foragers) make decisions. Neuroscientists have begun to study foraging decisions because of their ethological relevance and their unique ability to give a glimpse of decision-making as it was evolved to happen. Here we provide a brief introduction to the field, with a focus on two organisms selected to emphasize the breadth of foraging theory: nematodes and monkeys. Despite the obvious differences between these animals, it is clear that several basic principles, especially in the domain of regulation and control of sensory-motor transformations, apply to foraging decisions across taxa. These principles include the importance of the foreground/background structure in foraging decisions and the coordination of multiple input and output modalities to make beneficial long-term choices.