DNA polymerase β functions in DNA repair and has been extensively characterized as a model enzyme in order to ascertain the molecular mechanisms of DNA polymerase catalysis and fidelity. Research on the catalytic mechanism of polymerase β with a particular emphasis on substrate specificity is reviewed here. Notably, phosphodiester bond formation is the rate-limiting step of the reaction. Recent work has also provided strong evidence for an induced fit mechanism underlying correct selection of the nucleotide substrate for incorporation into DNA.