Obesity is an epidemic of the 21st century. The magnitude of this problem stems from its increasing prevalence and numerous metabolic complications caused by excessive fat accumulation. The pathogenesis involves both environmental and genetic factors, and FTO (fat mass and obesity-associated gene) is one of the most significant among genes predisposing to obesity. The role of FTO polymorphism in the development of obesity has been confirmed in many studies, but the effect varies significantly in different ethnic groups. Moreover, the exact mechanisms of FTO influence are yet to be explained. The association between FTO and lifestyle factors such as diet and physical activity has been extensively studied in recent years. This paper presents current knowledge about the role of FTO gene in the development of obesity and type 2 diabetes in different ethnic groups and the association between FTO polymorphism and lifestyle modifications predisposing to adiposity.