The current focus of nutritional science has shifted from meeting needs to determining the biological effects that nutrition has on immediate and lifetime health. Of particular interest is the concept of programming, the idea that "a stimulus or insult during a critical or sensitive period of development can have long-term or lifetime effects on an organism." Evidence that early nutrition has such "programming" effects in animals is overwhelming. In humans, retrospective observations show a relationship between adult disease and size in early life, though it is difficult to prove nutritional cause from observational associations and therefore difficult to use such data to underpin health policy. However, the results of randomized intervention trials of early nutrition with long-term follow-up are emerging. These experimental studies show that nutrition in early life has a major impact on health into early adulthood, notably on cardiovascular disease risk, bone health and cognitive function. These new findings have major biological, social and medical implications and should increasingly underpin health practices.