摘要：Summary Despite isolated areas of improvement, no country to date has reversed its obesity epidemic. Governments, together with a broad range of stakeholders, need to act urgently to decrease the prevalence of obesity. In this Series paper, we review several regulatory and non-regulatory actions taken around the world to address obesity and discuss some of the reasons for the scarce and fitful progress. Additionally, we preview the papers in this Lancet Series, which each identify high-priority actions on key obesity issues and challenge some of the entrenched dichotomies that dominate the thinking about obesity and its solutions. Although obesity is acknowledged as a complex issue, many debates about its causes and solutions are centred around overly simple dichotomies that present seemingly competing perspectives. Examples of such dichotomies explored in this Series include personal versus collective responsibilities for actions, supply versus demand-type explanations for consumption of unhealthy food, government regulation versus industry self-regulation, top-down versus bottom-up drivers for change, treatment versus prevention priorities, and a focus on undernutrition versus overnutrition. We also explore the dichotomy of individual versus environmental drivers of obesity and conclude that people bear some personal responsibility for their health, but environmental factors can readily support or undermine the ability of people to act in their own self-interest. We propose a reframing of obesity that emphasises the reciprocal nature of the interaction between the environment and the individual. Today's food environments exploit people's biological, psychological, social, and economic vulnerabilities, making it easier for them to eat unhealthy foods. This reinforces preferences and demands for foods of poor nutritional quality, furthering the unhealthy food environments. Regulatory actions from governments and increased efforts from industry and civil society will be necessary to break these vicious cycles.