Cancer epigenetics is rapidly moving into a translational phase, and knowledge on how aberrant DNA methylation is induced is becoming important. Aging, chronic inflammation, and viral infections are known to promote methylation of non-core regions of promoter CpG islands (CGI). The non-core methylation and 'seeds of methylation', scattered methylation in a CGI, are considered to serve as triggers for dense methylation of a promoter CGI, which permanently represses expression of its downstream gene. Decreased gene transcription is an important factor that promotes induction of dense methylation. The presence of the CGI methylator phenotype (CIMP), in which methylation of multiple CGI was observed, is under dispute. Some gastric cancer cell lines have increased rates of de novo methylation, and neuroblastoma cases with CIMP show qualitatively different prognosis from those without. This strongly supports the presence of CIMP, but it seems to contain multiple entities. Limited knowledge is available for epimutagens, the chemicals that induce DNA demethylation or methylation. We have developed an assay system to detect demethylating agents, and an assay system for methylating agents is necessary. Efforts in the field on how aberrant methylation is induced will lead to new cancer prevention, diagnostics, and therapeutics.