In any particular group of people-living in the same culture-some gain weight whilst others do not. Overconsumption of food is one factor contributing to this susceptibility to weight gain. Because all individuals are exposed to a similar range of environmental appetite-stimulating factors, the variability in overconsumption must be due to variability in intrinsic psychobiological processes. Such variability is an inevitable feature of living organisms. This essay explores whether susceptibility to weight gain is caused by variation in homeostatic processes-such as weak satiety responses to fat, or by hedonic processes-such as hyperresponsivity to the sensory properties of food. The question also arises whether the homeostatic or hedonic processes function separately and independently, or whether they interact. The answer to these questions can throw light upon the organization of behaviours associated with weight control, and can help to develop strategies to prevent weight gain. The theme of this essay was inspired by Gerry Smith's conceptual and experimental work on both homeostatic and hedonic mechanisms implicated in the control of food intake.