Human appetite is a complex mixture of physiological and psychological phenomena which include feelings of hunger, total energy intake, ingestion of particular nutrients, distribution and sizes of meals and snacks, specific cravings and food preferences. These phenomena can be assembled into a profile of motivation and a pattern of eating which represents the way in which appetite fluctuates over time. The satiety cascade shows the processes through which nutrition exerts effects on the biological system and, therefore, on feelings and behaviour. Within the biological system, the control of appetite involves post-ingestive mechanisms, including signals arising from the gastrointestinal tract and the release of hormones when food is processed. Post-absorptive mechanisms include the detection of important products of digestion, such as glucose and amino acids, together with the nature of the fuel mix oxidized and other metabolic variables. In obese patients, evidence points to a defect in the control of fat intake. In these people, dietary fat exerts only a weak action on satiation and satiety; it fails to generate strong responses in the mechanisms of the satiety cascade. An imbalance between fat intake and oxidation favours weight gain. A consideration of the psychobiological system (interactions between behaviour, peripheral physiology and neurochemical profiles) suggests strategies for treating or preventing the development of weight gain in vulnerable individuals.