Endometriosis is an estrogen-dependent inflammatory disease. In endometriotic tissues, a highestrogen environment associated with up-regulation of the aromatase gene has been well documented. There is accumulating evidence supporting a concept that endometriosis is a disease associated with an epigenetic disorder. Epigenetics is one of the most expanding fields in the current biomedical research. The word ‘epigenetics’ refers to the study of mitotically and/or meiotically heritable changes in gene expression that occur without changes in the DNA sequence. The disruption of such changes (epigenetic aberration or disorder) underlies a wide variety of pathologies. Epigenetic regulation includes DNA methylation and histone modifications, and is responsible for a number of gene transcription associated with chromatin modifications that distinguish the states of diseases. In this review, we summarized our studies as well as recent studies from other laboratories using an epigenetic approach focused on DNA methylation. We also summarized studies using advanced technologies including Genome-Wide (GW) methylation profiling analysis and GW Association Study (GWAS). We reviewed recent monozygotic twins studies in relation to environmental factors since they may provide insight into the epigenetic background of endometriosis. Finally, we referred to a new concept of GW DNA methylation.