DNA methylation, the addition of a methyl group at the 5-position of cytosine (5-methylcytosine, 5mC), is a major epigenetic modification in the genomes of many animals, plants, and fungi. In mammals, DNA methylation is involved in a variety of biological processes. Generally considered a stable chromatin mark, DNA methylation exhibits dynamic changes during mammalian development, including early embryogenesis and germ cell development. These changes are mediated by DNA methylation and demethylation machineries. These include DNA methyltransferases (DNMTs), which catalyze the transfer of a methyl group from the methyl donor S-adenosyl-l-methionine (AdoMet or SAM) to cytosine, and the recently characterized ten-eleven translocation (TET) family of 5mC dioxygenases, which oxidize 5mC and initiate demethylation. In this chapter, we will discuss the mechanisms of DNA methylation and demethylation in mammals, focusing on biochemical and genetic studies of the major players involved in these processes.