It has been proposed that there are critical periods during childhood that influence the development of obesity, including gestation and early infancy, the period of adiposity rebound that occurs between ages 5 and 7 years, and adolescence. Despite an extensive literature, there is to date only modest evidence for most of the factors such as nutrition, physical activity and other behavioural factors that are suspected as playing a role in the development of obesity. A recent review of this evidence (Parsons et al. 1999) showed, however, a consistent relationship between socio-economic status (SES) of origin and adult obesity, whereby those from lower SES backgrounds were fatter subsequently in adulthood. This association appeared to apply to both men and women, a finding that contrasts with the trends observed in cross-sectional studies, of an association with SES for women only. There are several potential explanations for the SES of origin-adult obesity relationship. SES of origin may be confounded by parental body size; studies to date provide insufficient evidence of an independent association with SES after allowing for parental body size. Alternatively, environment in early life (for which SES of origin is a proxy measure) may have a long-term impact on obesity later in adulthood, through one or more of several processes. Three major potential explanations can be identified: (1) nutrition in infancy and childhood, either over- or undenutrition, followed subsequently by overnutrition; (2) psychological factors, possibly involving emotional deprivation in childhood; (3) cultural or social norms regarding dietary restraint and attitudes to fatness that may be acquired during childhood.