Energy balance and macronutrient balance are the cornerstones upon which any theories of obesity must be built. Obesity can only occur when energy intake remains higher than energy expenditure for an extended period of time. However the macronutrient composition of the diet can also affect energy balance. Fat is a key nutrient because it is poorly regulated at both the level of consumption and oxidation. Psychological and behavioural profiles of obese subjects are clearly important because they can affect food choice and eating patterns. The role of eating frequency and circadian distribution of food is still debated. Eating disorders could be implicated in the development of obesity, but it is uncertain whether obesity is a direct result or a cause of the eating disorder. There are strong evidence to suggest that dietary restraint is associated with loss of dietary control and excessive eating. Early stages of fat storage involve expansion of existing adipocytes (hypertrophy) and later stages involve the recruitment of new adipocytes (hyperplasia). The mechanisms controlling the transformation of preadipocyte could also involve specific dietary components such as polyunsaturated fatty acids or proteins. The age of adiposity rebound, that is a risk factor for later obesity has been found significantly younger in children consuming a high protein diet. These factors could be involved during early infancy or even in utero, according to the hypothesis of fetal programming of adult diseases. There is a need for more longitudinal studies on the role of macronutrient composition, food choice or eating disorders, especially among children, teenagers and young adults.