The prevalence of obesity is increasing worldwide, which indicates that the primary cause of obesity lies in environmental and behavioural changes rather than in genetic modifications. Among the environmental influences, the percentage of fat energy of the everyday diet and the lack of physical activity are two important factors, which contribute to explain the rising prevalence of obesity. In this review, several lines of evidence are presented to illustrate why dietary fat does affect obesity development. There are four factors which support a link between dietary fat and obesity development:The thermic effect of nutrients, expressed as percentage of their energy content, is 2-3% for lipids, 6-8% for carbohydrates and 25-30% for proteins. This means that the efficiency of nutrient utilization (calculated as 100%-the thermic effect of the nutrient) is higher for fat than for carbohydrate or protein.Postingestive fuel selection favours the oxidation of dietary proteins and carbohydrates, whereas dietary fats are preferentially stored as triacylglycerol in adipose tissue. Alcohol, by inhibiting lipid oxidation, indirectly favours the storage of dietary fats.High-fat diet promotes excessive energy intake by passive overconsumption; the fat-induced appetite control signals are too weak or too delayed to prevent excessive energy intake from a fatty meal. The only proof that dietary fats contribute to weight gain is to test the long-term effect of ad libitum low-fat diets. Most studies on low-fat diets show that they induce a modest weight loss in obese individuals, but their long-term effect from a public health perspective is limited, probably due to a low compliance to the dietary advice.