Globally, the incidence of obesity is increasing, which is leading to challenges for health-care systems. Despite the rising incidence of obesity, levels in those aged 15–19 years have remained fairly stable at <5%. This age range therefore represents an important window of opportunity to prevent obesity in adulthood. If young people can be encouraged to engage with and understand the complex issues underlying obesity, the rising prevalence of obesity could be halted, and even reversed. In this Perspectives article, Michael Lean and colleagues provide examples of social movements led by young people (often using social media and other web-based platforms) and suggest that similar movements could be fostered to prevent young people developing obesity in adulthood.