Obesity, as in every western country, is currently the most prevalent chronic disease in childhood in Spain. This has led to obesity being one of the most common consultations in general paediatrics and, particularly, in paediatric endocrinology. Furthermore, obesity associated comorbidities are increasing in prevalence in children and adolescents. It is widely accepted that this increase in the prevalence of obesity is derived from an imbalance between energy intake and expenditure, associated to the lifestyle in western countries. However, there is increasing evidence of the role of individual and familial genetic background in the risk of developing obesity. The pathophysiological basis of the mechanisms responsible for the control of appetite and energy expenditure are being discovered on the basis of the increasing known cases of human monogenic, syndromic and endocrine obesity. Thus it is no longer appropriate to talk about obesity but rather about «obesities», as their pathophysiological bases differ and they require different diagnostic and management approaches. In 2011, the paediatrician must be aware of this issue and focus the clinical history and physical examination towards these specific clinical sign and symptoms, to better manage the available diagnostic and therapeutic resources when faced with a child with obesity.