DNA methylation, the addition of a methyl group to the carbon-5 position of cytosine residues, is the only common covalent modification of human DNA and occurs almost exclusively at cytosines that are followed immediately by a guanine (so-called CpG dinucleotides). The bulk of the genome displays a clear depletion of CpG dinucleotides, and those that are present are nearly always methylated. By contrast, small stretches of DNA, known as CpG islands, are comparatively rich in CpG nucleotides and are nearly always free of methylation. These CpG islands are frequently located within the promoter regions of human genes, and methylation within the islands has been shown to be associated with transcriptional inactivation of the corresponding gene. Alterations in DNA methylation might be pivotal in the development of most cancers. In recent years, it has become apparent that the pattern of DNA methylation observed in cancer generally shows a dramatic shift compared with that of normal tissue. Although cancers often exhibit clear reductions throughout their genomes in the levels of DNA methylation, this goes hand-in-hand with increased methylation at the CpG islands. Such changes in methylation have a central role in tumourigenesis; in particular, methylation of CpG islands has been shown to be important in transcriptional repression of numerous genes that function to prevent tumour growth or development. Studies of DNA methylation in cancer have thus opened up new opportunities for diagnosis, prognosis and ultimately treatment of human tumours.