Classical genetics has revealed the mechanisms for the transmission of genes from generation to generation, but the strategy of the genes in unfolding the developmental programme remains obscure. Epigenetics comprises the study of the mechanisms that impart temporal and spatial control on the activities of all those genes required for the development of a complex organism from the zygote to the adult. Epigenetic changes in gene activity can be studied in relation to DNA methylation in cultured mammalian cells and it is also possible to isolate and characterize mutants with altered DNA methylase activity. Although this experimental system is quite far removed from the epigenetic controls acting during development it does provide the means to clarify the rules governing the silencing of genes by specific DNA methylation and their reactivation by demethylation. This in turn will facilitate studies on the control of gene expression in somatic cells of the developing organism or the adult. The general principles of epigenetic mechanisms can be defined. There are extreme contrasts between instability or switches in gene expression, such as those in stem-line cells, and the stable heritability of a specialized pattern of gene activities. In some situations cell lineages are known to be important, whereas in others coordinated changes in groups of cells have been demonstrated. Control of numbers of cell divisions and the size of organisms, or parts of organisms, is also essential. The epigenetic determination of gene expression can be reversed or reprogrammed in the germ line. The extent to which methylation or demethylation of specific DNA sequences can help explain these basic epigenetic mechanisms is briefly reviewed.