Publisher Summary This chapter describes DNA polymerases of mesophiles and covers DNA polymerases of thermophilic Eubacteria and Archaea. It covers both thermophiles, organisms that grow optimally in the range of 50–70oC, but not at 90oC, and hyperthermophiles, organisms with an optimal growth temperature of at least 80°C, which also grow at 90°C. The most highly studied DNA polymerase is the Pol I DNA polymerase from Escherichia coli, and the truncated version of this enzyme, called the Klenow fragment, includes the C-terminal portion of E.coli Pol I containing the 3'→ 5'exonuclease and polymerase domains. Besides adding nucleotides to a growing DNA chain (polymerization), a DNA polymerase may also remove nucleotides from a DNA chain. This removal of nucleotides is referred to as the exonuclease activity of a DNA polymerase, and it can proceed in either the 3' to 5' direction (3'→ 5' exonuclease) or the 5' to 3' direction (5'→ 3'exonuclease). Proofreading is the term applied to the 3'→5'exonuclease activity of a DNA polymerase when it excises a newly synthesized mismatch. This chapter examines only the properties of the DNA polymerase catalytic polypeptide and discusses its structures and types.