Childhood asthma is linked strongly to atopy and is characterised by a T helper 2 (Th2)-polarised immunological response. Epidemiological studies implicate severe lower respiratory tract viral infections, especially in early childhood, and repeated inhalational exposure to allergens as important synergistic factors in the development of asthma. The way in which these and other environmental factors induce stable alterations in phenotype is poorly understood, but may be explained on the basis of epigenetic changes, which are now recognised to underlie the establishment and maintenance of a Th2 response. Furthermore, ongoing asthmatic inflammation of the airways may be driven by alterations in the expression profile of regulatory microRNA genes, to which epigenetic mechanisms may also contribute. Thus, an understanding of epigenetic mechanisms in asthma has the potential to reveal new approaches for primary prevention or therapeutic intervention in childhood asthma.