The emerging field of epigenetics provides a new frontier for understanding mechanisms underlying well-recognized gene-environment interactions. Epigenetics refers to the study of heritable changes in gene expression, caused by chemical modifications in a chromosome, which are so termed because they alter the likelihood of gene transcription but do not change the underlying sequence. This particular mechanism of gene regulation is fundamental to all aspects of growth and development, determining the flow of genomic information in a temporal and tissue-specific manner. The epigenetic program is encoded by specific histone modifications (methylation and acetylation) and DNA methylation patterns, which determine the degree of DNA compaction and thus the accessibility of genes for transcription (Fig 1). In addition to histone modifications and DNA methylation, there are also other gene regulatory networks, including microRNAs, small interfering RNAs, and long noncoding RNAs, all of which serve to control gene expression.1 However, these are distinct from epigenetic traits as it is unclear whether these processes are transmittable across generations. During cellular differentiation, these networks selectively alter the patterns of gene expression to allow considerable cellular diversity while the DNA code remains unchanged. In a general sense the epigenetic code provides plasticity of gene expression in response to environmental changes, allowing more rapid phenotypic adaptations across generations. As developmental changes are most profound during embryonic and fetal life, this is also arguably when the epigenetic program is most vulnerable to environmental changes. Given that these modifications are reversible and sensitive to environmental factors, they provide a mechanistic link between environmental exposures, developmental programming, and risk for disease.