Obesity is an environmental issue. Societies that are transitioning to westernized lifestyles are experiencing substantial increases in its prevalence. The primary environmental determinants of obesity are high calorie intake and low levels of activity. Socioeconomic status and place of residence are important contributors. These factors together comprise an obesogenic or 'toxic' environment where the development of obesity is the expected course for humans leading lifestyles incompatible with their evolutionary development. Only by addressing and modifying the toxic environment will we be able to stem the obesity epidemic.