Obesity is a complex chronic disease in which abnormal or excess body fat (adiposity) impairs health, increases the risk of long-term medical complications and reduces life span. However, due to individual differences in body composition, body fat distribution and function, the threshold to which adiposity impairs health is highly variable among adults.1 Epidemiological and population studies define obesity using the body mass index (BMI, weight/height2 ). BMI is a fairly reliable anthropometric measurement to stratify obesity-related health risks at the population level. Obesity is operationally defined as a BMI exceeding 30 kg/m2 , and is subclassified into Class I (BMI 30–34.9), Class II (BMI 35–39.9) and Class III (BMI > 40). Obesity is a chronic disease caused by the complex interplay of genetic, metabolic, behavioural and environmental factors; the latter are thought to be the proximal cause of the dramatic rise in the prevalence of obesity.2 The increased availability of processed, affordable and effectively marketed food, abundance of sugar-sweetened beverages, economic growth, behavioural changes and rapid urbanization in lowand middle-income countries are some of the key drivers that promote overconsumption of food.3 With respect to energy expenditure, the level of physical activity for leisure has been relatively stable or slightly elevated over the last 50 years.4 This chapter attempts to address the cellular and molecular pathogenesis of obesity to inform a rational approach to management of this complex disease.