Obesity rates express the proportion of a population classified as obese, usually defined as a body mass index (BMI) of 30 or above, and are the headline indicator of the global obesity epidemic. They are tracked through national health surveys using measured, not self-reported, height and weight, and published by bodies such as the WHO, OECD, and national statistics agencies. Rates have risen in nearly every country over recent decades, with severe obesity growing fastest, and they vary starkly by income, education, ethnicity, and neighbourhood. Policymakers use the figures to justify interventions from sugar taxes to urban planning, while researchers link trends to changes in food environments, portion sizes, and physical activity. The measure has limits, since BMI misclassifies muscular individuals, but at population scale it remains the standard surveillance tool.