It is recognized that obesity is a multifactorial condition, in other words it is caused by more than one determinant:

- genetic factors: for some time now scientific research has been focusing its efforts on studying the genetics of obesity as well as appetite controlling mechanisms and energy metabolism. It is likely that combined genetic factors exist which are responsible for a persons capacity or incapacity to lose weight and keep weight down, however, known cases of genuine genetically inherited obesity are still very rare.

- social-environmental factors and sedentary lifestyle: the commonest cause of obesity and overweight is an energy imbalance between the number of calories consumed in foods and those burned off. In general it is linked to an increase in the consumption of high calorie foods rich in fats and sugars and a reduction in physical activity, partly associated with an increase in sedentary lifestyles.

- psychological factors: often there are pathological forms of hyperalimentation which can be brought about by stress or emotional disorders, in some cases there may be very real eating disorders such as Night Eating Syndrome and certain forms of compulsive eating (binge eating, etc.)

- pharmacological causes: certain drugs may contribute to the development of obesity by increasing hunger levels, reducing metabolism or stimulating the proliferation of adipose cells. Anti-depressants and corticosteroids are amongst those types of drug most frequently associated with obesity.

- endocrine and metabolic causes: there are certain rare hormonal disorders which can cause obesity, such as, for example, conditions where the body produces excessive amounts of cortisol.

- brain damage: there are very rare conditions in which brain damage to the hypothalamus region (area of the brain involved in appetite control) can lead to obesity.