The worldwide increase in obesity and related chronic diseases has largely been driven by global trade liberalization, economic growth and rapid urbanization. These factors continue to fuel dramatic changes in living environments, diets and lifestyles in ways that promote positive energy balance. Nutritional transitions in low-income and middle-income countries are typically characterized by increases in the consumption of animal fat and protein, refined grains, and added sugar. This change is coupled with reductions in physical activity owing to more mechanized and technologically driven lifestyles. Given the high costs of obesity and comorbidities in terms of health-care expenditure and quality of life, prevention strategies are paramount, particularly in low-income and middle-income countries that must manage coexisting infectious diseases and undernutrition in addition to the obesity epidemic. As countries become increasingly urbanized, undernutrition and obesity can exist side by side within the same country, community or household, which is a particular challenge for health systems with limited resources. Owing to the scope and complexity of the obesity epidemic, prevention strategies and policies across multiple levels are needed in order to have a measurable effect. Changes should include high-level global policies from the international community and coordinated efforts by governments, organizations, communities and individuals to positively influence behavioural change.