Obesity: a bio-behavioural-environmental phenomenon: Obesity is on the increase all over the world in technologically advanced countries, developing countries and rural communities. What is causing this upward drift in body weight? Can drugs do anything to ameliorate the situation? It is generally agreed that obesity results from genetic vulnerability combined with a provocative environmental situation. This provides the basis for a psychobiological interaction in which behaviour plays a key role. This is the case since it is behaviour which translates biological propensities into action on the environment, and it is behaviour which mediates (in part) the effect of the environment upon biology. Two particularly important behavioural aspects of the genes-environment interaction are low levels of physical activity (high sedentariness) and dietary habits which favour overconsumption (high intake of energy, particularly as fat). One continuing theme of research is the development of drugs to allow people to gain control over appetite by modifying eating patterns (dietary habits) through a number of possible mechanisms. The use of drugs to make people more willing or more able to engage in physical activity is not widely discussed although the use of drugs to increase total energy expenditure (via a variety of mechanisms) is actively researched. More than a decade ago Sullivan defined the framework for the development of anti-obesity drugs by specifying that drugs could act on energy intake, energy output or on those mechanisms involved in the assimilation and storage of lipids in the body.(ABSTRACT TRUNCATED AT 250 WORDS)