DNA methylation is a heritable epigenetic modification of cytosine residues within CpG dinucleotides associated with the modulation of gene expression and found at 20-30 million sites throughout the mammalian genome. The methylation of DNA is catalyzed by DNA (cytosine-5)-methyltransferases (DNMTs), regulates genes during development and plays a role in genomic imprinting and X inactivation. Abnormalities in DNA methylation lead to growth and behavioral abnormalities as well as cancer. Genomic methylation patterns are acquired in the germline and then further modified during embryogenesis. Methylation in the male germline is unique in comparison to that in somatic tissues, begins in the fetal gonad before birth, and is completed during postnatal spermatogenesis. In rodents, altered expression of the DNMTs through gene-targeting or drug treatment is associated with abnormal DNA methylation patterns in germ cells and perturbations in spermatogenesis. In man altered sperm DNA methylation patterns have been reported in individuals with oligospermia.