Developmentally programmed global loss of 5-methylcytosine (5mC) involves the TET1 and TET2 enzymes that convert 5mC to 5-hydroxymethylcytosine (5hmC). TET enzyme function has been investigated in embryonic stem cells, and Azim Surani and colleagues now report a functional investigation of 5hmC in mouse primordial germ cells (PGCs) (Science 339, 448–452, 2013). The authors isolated PGCs and profiled genomic patterns of 5mC and 5hmC. They showed that Tet1 and Tet2 are expressed in PGCs at the time of loss of 5mC and gain of 5hmC and that 5mC erasure is coupled to 5hmC enrichment. They found that, after the peak of 5hmC conversion, the levels of 5hmC were progressively depleted, indicating a process of replication-coupled dilution. The authors created an inducible system for the double knockdown of Tet1 and Tet2 in cultured PGC-like cells; this resulted in inhibition of 5mC demethylation. Conversely, overexpression of Tet1 and Tet2 promoted 5mC erasure. Whole-genome bisulfite sequencing showed that a number of loci, mostly repeat elements, escape demethylation. This work establishes a role for TET enzymes and 5hmC in the process of DNA demethylation in developing primordial germ cells.