Diet is of major interest in research on the etiology of obesity. Research in this field comprises investigation of the role of individual nutrients and foods, nutrient composition, as well as dietary patterns and habits. Longitudinal data on the association between dietary factors and the development of obesity in childhood and adolescence are sparse; therefore, conclusions on the impact of energy density, consumption of carbohydrates and proteins, snack foods and fast food, meal patterns and speed of eating cannot be drawn. More data exist with respect to the role of energy intake and consumption of fat and sugar-sweetened beverages; however, findings are inconsistent. This could be due to methodological shortcomings that mark dietary assessment in children and adolescents. However, as a direct modulator of energy balance, diet still needs to be part of a comprehensive strategy to combat overweight and obesity in children and adolescents.