The article discusses the role of ten eleven translocation (TET) enzymes in DNA methylation, development, and cancer. TET enzymes oxidize 5-methylcytosines (5mCs) and promote the reversible reversal of DNA methylation. The *TET* genes, particularly *TET2*, are frequently mutated in various cancers, but their exact mechanisms in preventing cancer onset and maintenance are not fully understood. The article highlights recent advances in understanding the physiological functions of TET proteins and their role in regulating DNA methylation and transcription. It also discusses key outstanding questions in the field, such as the dual role of TET enzymes in maintaining DNA methylation fidelity and executing regulatory functions at specific genomic sites. The article further explores the impact of TET proteins on DNA methylation in vivo, the recruitment and regulation of TET proteins to DNA, and their physiological functions in development and disease. Finally, it reviews the role of TET proteins in hematological cancers, emphasizing the frequent mutations in *TET2* and their potential as early genetic aberrations in the onset of hematological malignancies.The article discusses the role of ten eleven translocation (TET) enzymes in DNA methylation, development, and cancer. TET enzymes oxidize 5-methylcytosines (5mCs) and promote the reversible reversal of DNA methylation. The *TET* genes, particularly *TET2*, are frequently mutated in various cancers, but their exact mechanisms in preventing cancer onset and maintenance are not fully understood. The article highlights recent advances in understanding the physiological functions of TET proteins and their role in regulating DNA methylation and transcription. It also discusses key outstanding questions in the field, such as the dual role of TET enzymes in maintaining DNA methylation fidelity and executing regulatory functions at specific genomic sites. The article further explores the impact of TET proteins on DNA methylation in vivo, the recruitment and regulation of TET proteins to DNA, and their physiological functions in development and disease. Finally, it reviews the role of TET proteins in hematological cancers, emphasizing the frequent mutations in *TET2* and their potential as early genetic aberrations in the onset of hematological malignancies.