DNA methylation is a stable and heritable epigenetic mark, and it is one of the best characterized epigenetic modifications. Active DNA demethylation has been reported both in plant and animal cells, and the mechanism behind it is becoming clear in plant. Whereas a bona fide enzyme, which is responsible for active DNA demethylation, have not been identified in mammals, and active demethylation pathway is controversial. In the present review, we described that active DNA demethylation take place in a spatial- and temporal-specific way on the basis of recent literatures. Moreover, several candidate pathways such as oxygenation and deamination of 5-methyl cytosine and DNA repair pathways, which may be responsible for the active process were introduced on a cell- and tissue-specific view. The aim of this paper is to help re-searchers reveal the mechanism underlying this important event during epigenetic reprogramming in mammals.