The rapid increase in the prevalence of obesity in children and adolescents raises concerns about the main determinants of this phenomenon. From a life course perspective, determinants of obesity development appear early in life (Fig. 26.1). Depending on the timing and the complex interplay of multiple exposures, the excess of body fat and obesity related complications, mainly the metabolic ones, will develop. Obesity and its related complications are responsible for, in the short and long term, chronic morbidity, including cardiovascular diseases, type 2 diabetes and cancer, amongst others, and result in excess mortality later in life (Fig. 26.1). For these reasons, primary prevention of obesity beginning in childhood or even earlier during pregnancy should be a public health priority.