Obesity is characterised by alterations in metabolic function which result from a combination of increasing total body fatness and the regional distribution of adipose tissue. Abdominal visceral obesity is particularly associated with hyperinsulinaemia, increased portal vein free fatty acid concentration, hepatic gluconeogenesis, altered adrenocortical activity and androgen secretion and reduced plasma sex hormone binding globulin levels. These alterations, which are accompanied by changes in visceral adipocyte sensitivity to plasma catecholamine stimulation, enhance further visceral fat deposition and the perpetuation of the metabolic derangements. The characteristic dyslipidaemia associated with upper body obesity and the frequent development of NIDDM are predictable consequences. In contrast to the considerable knowledge about the biochemical background to these alterations, relatively little is understood about the mechanisms through which an individual's ethnic background influences the changes. This chapter reviews these important issues.