In vivo the DNA polymerases are responsible for replicative and repair DNA synthesis. These enzymes use the pre-existing 3'-OH group of a primer annealed to a single-stranded DNA template to incorporate monophosphate deoxynucleosides (dNMPs) in a sequential and directional manner. Although all DNA polymerases share a similar catalytic core constituted by a palm, a thumb and a fingers domain and a similar chemical mechanism of dNMP incorporation that requires two metal cations, they intrinsically differ by the nature of the step that controls the incorporation of dNMP and by their capacity to cope with lesions. Several factors, such as the size of the active site, the flexibility of the DNA in the active site or the presence of protein subdomains devoid of known catalytic activity but able to accommodate small DNA loops, control the fidelity of DNA polymerases. Auxiliary replication factors, such as the processivity factor or the single-stranded DNA binding protein, can also modulate the intrinsic properties of DNA polymerases and therefore fine-tune the.