The construction of a variety of strains deficient in the methylation of adenine and cytosine residues in DNA by the methyltransferases (MTases) Dam and Dcm has allowed the study of the role of these enzymes in the biology of Escherichia coli. Dam methylation has been shown to play a role in coordinating DNA replication initiation, DNA mismatch repair and the regulation of expression of some genes. The regulation of expression of dam has been found to be complex and influenced by five promoters. A role for Dcm methylation in the cell remains elusive and dcm- cells have no obvious phenotype. dam- and dcm- strains have a range of uses in molecular biology and bacterial genetics, including preparation of DNA for restriction by some restriction endonucleases, for transformation into other bacterial species, nucleotide sequencing and site-directed mutagenesis. A variety of assays are available for rapid detection of both the Dam and Dcm phenotypes. A number of restriction systems in E. coli have been described which recognise foreign DNA methylation, but ignore Dam and Dcm methylation. Here, we describe the most commonly used mutant alleles of dam and dcm and the characteristics of a variety of the strains that carry these genes. A description of several plasmids that carry dam gene constructs is also included.