The epigenetic memory stored in the dynamic modifications, such as base modifications of cytosine (C) in DNA, including methylation/hydroxymethylation/demethylation, causes heritable phenotypes via regulating gene expression without alteration of DNA sequence. The process from cytosine modification to the epigenetic effect is orchestrated by complicated machinery consisting of writers, erasers, readers, and other factors. The two major forms of cytosine modification include methylcytosine (5-mC) and hydroxymethylcytosine (5-hmC). DNA methyltransferases (DNMTs) including DNMT1, DNMT3A, and DNMT3B function as writers for 5-mC. The ten-eleven translocation proteins (TET) including TET1, TET2, and TET3 in the mammalian genome are responsible for hydroxymethylation of 5-mC to generate 5-hmC, 5-formylcytosine (5-fC), and 5-carboxylcytosine (5-caC). The 5-mC and 5-hmC have become the two most extensively investigated epigenetic markers, and the dynamic balance of these two markers shape the landscape of the epigenome, functioning as a platform to regulate gene expression epigenetically. The landscape of the 5-hmC in epigenome is precisely and tightly regulated during the development. Aberrant alterations of the epigenetic regulation may cause severe consequences such as phenotype change as well as initiation of disease. Progressively, significant achievements have been made in characterization of writers, erasers, and readers of 5-mC and 5-hmC, as well as the contribution of aberrant alteration of 5-hmC/5-mC landscape to the pathogenesis of human diseases, such as cancers and neurological disorders. This article will highlight the research advances in the distinct contribution of TET proteins as suppressors or promoters to the pathogenesis of tumorigenesis and progression. Furthermore, this article also discusses the challenges and the directions for research in the future.