Epigenetics is a postmeiotic modification of gene expression that is independent of the primary DNA sequence. DNA methylation, methylated DNA binding proteins, and histone modification-related enzymes are associated with epigenetics. Abnormalities in DNA methylation of CpG islands which are important for gene expression control, affect gene expression, which may influence carcinogenesis, aging, and other diseases. Aberrant DNA methylation occurs with aging, inflammation, viral infection, and carcinogenesis. DNA methylation can be evaluated for molecular analysis for diagnosis of early cancer. It is also important for laboratory diagnosis by using classic and authentic laboratory tests because the tests can be affected by epigenetics-controlled gene expression. It is also related to the effectiveness of therapeutic agents affecting DNA methylation and histone deacetylation, and the strategy in search of genetic abnormality for epigenetic as well as genetic error.