PURPOSE OF REVIEW: Is trafficking of dietary fat involved in the development of obesity? RECENT FINDINGS: Studies on energy expenditure and substrate utilization during overfeeding, studies on individual differences in substrate utilization between individuals fed at energy balance, and differences in responses in energy expenditure and substrate utilization in individuals after weight reduction, illustrate that the capacity of fat oxidation is a potential determinant for the development of obesity. SUMMARY: The ability to store dietary fat seems to be involved in the susceptibility to gain weight during a positive energy balance. Obese individuals show less oxidation and more storage of dietary fat as compared with the lean phenotype. Differences in fuel trafficking make individuals prone to overeating in the current obesogenic environment with a high availability of energy-dense fatty foods. It is difficult to get rid of excess body fat as energy requirement for weight maintenance after weight reduction is lower than predicted from the new body composition reached and thus, there is a high risk for weight regain.