Overweight and obesity are currently the most important health problems of humanity. Weight gain begins in early childhood, and more precisely, in the pregestational period and at 1000 days of life, which is the most important period for preventing overweight and obesity in adulthood. Obesity is a multifactorial condition that is influenced by maternal condition, delivery mode, type of feeding, obesogenic diet, parents’ level of education, physical activity, and socioeconomic and genetic factors. This chapter discusses the main risk factors for overweight and obesity in children in the pregestational period, during pregnancy, and postnatal periods. The following are also discussed: the multicomponent and multilevel intervention; the characteristics of the gut microbiota in children with overweight and obesity; the effects of the interventions with probiotics and prebiotics; the interventions to prevent overweight and obesity; the treatment for overweight and obesity in children; and the actual guidelines and recommendations that lead to the prevention of these conditions.