Obesity represents a major threat to health and quality of life. Although obesity has strong genetic determinants, it is generally accepted that it results from an imbalance between food intake and daily physical activity. Health guidelines have been focused on 3 particular lifestyle factors: increased levels of physical activity and reductions in the intakes of fat and sugars. The dietary guidelines, especially, are under debate. This review covers evidence from carefully controlled laboratory studies, clinical trials, studies in populations at high risk of developing obesity, and epidemiologic studies on the role of sugars, particularly sucrose, in the development of obesity. Although many environmental factors promote a positive energy balance, it is clear that the consumption of a low-carbohydrate, high-fat diet increases the likelihood of weight gain. The evidence related to carbohydrate, particularly sugars, and the type of food (solid or liquid) is less clear because the number of long-term ad libitum dietary intervention trials is very small. Data on sucrose intake in relation to metabolism and weight gain do not associate high consumption of sucrose with the prevalence of obesity. The evidence supports the current dietary guidelines for reducing fat intake. However, the effect of the carbohydrate source and class and of the form in which carbohydrate is consumed (solid or liquid) on body weight control requires further consideration.