A small number of mammalian genes undergo the process of genomic imprinting whereby the expression level of the alleles of a gene depends upon their parental origin. In the past year, attention has focused on the mechanisms that determine parental-specific expression patterns. Many imprinted genes are located in conserved clusters and, although it is apparent that imprinting of adjacent genes is jointly regulated, multiple mechanisms among and within clusters may operate. Recent developments have also refined the timing of the gametic imprints and further defined the mechanism by which DNA methyltransferases confer allelic methylation patterns.