5-Methylcytosine (5mC) can be converted to 5-hydroxymethylcytosine (5hmC) in mammalian cells by the ten-eleven translocation (Tet) family of dioxygenases. While 5mC has been extensively studied, we have just started to understand the distribution and function of 5hmC in mammalian genomes. Despite the fact that this new epigenetic mark has only been discovered three years ago, exciting progress has been made in understanding its generation, fate, and genomic distribution. In this review we discuss these progresses as well as the recent advance in the single-base resolution mapping of 5hmC.