The developmental origins of health and disease can be understood by reference to the fundamentals of developmental plasticity. It is essential to distinguish between those environmental effects acting during development that are disruptive from those that have adaptive value. The latter are likely to underpin programming and the developmental origins of adult disease. It is suggested that greater disease risk is created by a mismatch between the environment predicted during the plastic phase of development and the actual environment experienced in the postplastic phase. This plastic phase extends from conception to after birth at least for some systems. It is not necessary to invoke a particular mechanism in the neonatal or infant period. There is increasing evidence that prematurity can be associated with long-term consequences, and this is to be anticipated from conceptual considerations. Different preventative strategies may be relevant in different populations.