The number of overweight children is increasing rapidly, and there is an urgent need to identify the risk factors for obesity, with a view to preventing further increases in prevalence. Experimental studies in animals and preliminary observations in humans suggest that early experience may influence later risk of obesity, but we do not yet understand the extent to which early influences affect individual vulnerability to risk factors acting later in life. In the developed world, few studies have examined whether current variations in maternal diet have long-term effects on children's body composition. Rapid postnatal growth is associated with greater adiposity, but the role of variations in infant diet and the mechanisms involved are not understood, and there may be interactive effects of early diet and growth rate on body composition. Familial concordance in obesity prevalence suggests that the shared food environment is of key importance. Early life may be a time when dietary practices are established that will continue throughout childhood. Further research is needed to gain insight into the evolution of dietary habits in childhood and to determine how these practices influence obesity risk.