DNA methyltransferases (MTases) uniquely combine the ability to recognize and covalently modify specific target sequences in DNA using the ubiquitous cofactor S-adenosyl-L-methionine (AdoMet). Although DNA methylation plays important roles in biological signaling, the transferred methyl group is a poor reporter and is highly inert to further biocompatible derivatization. To unlock the biotechnological power of these enzymes, two major types of cofactor AdoMet analogs were developed that permit targeted MTase-directed attachment of larger moieties containing functional or reporter groups onto DNA. One such approach (named sequence-specific methyltransferase-induced labeling, SMILing) uses reactive aziridine or N-mustard mimics of the cofactor AdoMet, which render targeted coupling of a whole cofactor molecule to the target DNA. The second approach (methyltransferase-directed transfer of activated groups, mTAG) uses AdoMet analogs with a sulfonium-bound extended side chain replacing the methyl group, which permits MTase-directed covalent transfer of the activated side chain alone. As the enlarged cofactors are not always compatible with the active sites of native MTases, steric engineering of the active site has been employed to optimize their alkyltransferase activity. In addition to the described cofactor analogs, recently discovered atypical reactions of DNA cytosine-5 MTases involving non-cofactor-like compounds can also be exploited for targeted derivatization and labeling of DNA. Altogether, these approaches offer new powerful tools for sequence-specific covalent DNA labeling, which not only pave the way to developing a variety of useful techniques in DNA research, diagnostics, and nanotechnologies but have already proven practical utility for optical DNA mapping and epigenome studies.