Following Barker's observations of an association between birth size and later adult diseases, considerable efforts have been made to define the characteristics of low birth weight groups in childhood. In this review, the phenotypic and biochemical characteristics during childhood of three low birth weight groups are summarized: children born following inviter fertilization (IVF), small for gestational age (SGA), or very premature. Each of these groups is likely to have been exposed to an adverse environment at different developmental stages. The triggers and mechanisms leading to programmed changes in growth, development, and metabolism of these groups of children have yet to be identified. Epigenetics has been proposed as a potential mechanism for these programmed changes through environmentally induced changes in gene expression. Data from animal models in which environmental, particularly nutritional, manipulation leads to changes in DNA methylation are presented. The relevance of these animal studies to IVF, SGA, and very premature children are discussed as are potential candidate genes that may have undergone epigenetic modification to alter growth and metabolism.