DNA methyltransferases are important enzymes and their inhibition has many potential applications. The investigation of DNA methyltransferases as well as screening for potential inhibitors requires specialized enzyme assays. In this chapter, we describe three DNA methyltransferase assays, each of them based on a different method: (1) An assay using radioactively labeled AdoMet and biotinylated DNA substrates that is ideal for enzymatic characterization of these enzymes. (2) An assay using bisulfite conversion of in vitro methylated DNA that is ideal to determine details of the methylation pattern introduced by DNA-(cytosine C5)-methyltransferases. (3) A novel fluorescence-coupled, restriction-based assay suitable for high-throughput screening of DNA methyltransferase inhibitors.