Obesity is not a single disorder but a heterogeneous group of conditions with multiple causes each of which are ultimately expressed as an obese phenotype. Fatness does run in families, but the genetic component does not follow simple Mendelian principles and the influence of the genotype on the aetiology of obesity may be attenuated or exacerbated by non-genetic factors. Body weight is ultimately determined by the interaction of genetic, environmental and psychosocial factors acting through the physiological mediators of energy intake and expenditure.