The recent crystal structure determination of T7 DNA polymerase complexed to a deoxynucleoside triphosphate and primer-template DNA has provided the first glimpse of a replicative DNA polymerase in a catalytic complex. The structure complements many functional and structural studies of this and other DNA polymerases, allowing a detailed evaluation of proposals for the mechanism of nucleotidyl transfer and the exploration of the basis for the high fidelity of template-directed DNA synthesis.