Publisher Summary The chapter discusses on the DNA methylation and its possible biological roles. The pattern of DNA methylation is species-specific. In the DNA of some prokaryotes, a small fraction of the cytosine residues is methylated, while in others only adenine residues are methylated. A third group of prokaryotic organisms contain both 5-methylcytosine (m5Cyt) and N6-methyladenine (m6Ade) in their DNA. Eukaryotic DNA, in general, is methylated exclusively at cytosine residues. The species-specific patterns of methylation reflect base specificity as well as sequence specificity of the methylases involved. The introduction of restriction enzymes and DNA cloning techniques into this field of research has already proved to be very fruitful, and should provide sufficient tools for the elucidation of the function of methyl groups in DNA. Tissue specificity with respect to the pattern of methylation can be observed when specific methylatable sites in single genes are studied. However, no significant differences are noted when the average methylation of DNA from various tissues is studied. Preliminary results of experiments in bacteria demonstrate that methylase-deficient mutants are hypersensitive to mutagenesis, and have higher rates of DNA recombination in the chapter, are also preliminary indications that DNA methylation is coupled to DNA replication in E. coli. These experiments should be extended to eukaryotic systems.