DNA demethylation is necessary for the epigenentic reprogramming of genes and is also directly involved in many important disease mechanisms such as tumor progression. Demethylation of DNA can either be passive or active, or a combination of both. Passive DNA demethylation usually takes place on newly synthesized DNA strands via DNMT1 during replication rounds. Active DNA demethylation mainly occurs by the removal of 5-methylcytosine through further modified cytosine bases which have been converted by TET enzyme-mediated oxidation. These oxidation products have been shown to be repaired by TDG, a glycosylase which is involved in base excision repair, or directly converted to cytosine by DNMT3A/ DNMT3B in oxidative states. It is proposed that DNA demethylation could also be initiated by deamination of 5-mC through candidate deaminases including AID and APOBEC1, which convert 5-mC to thymine. The resulting thymine could be repaired by BER initiated by a T-G mismatch glycosylase such as MBD4 or TDG. In addition, the 5-mC base can be directly removed in plants by the DME/ROS1 family of 5-mC DNA glycosylases, resulting in an abasic site that is repaired by the BER process.