Underpinned by a database of more than a dozen different crystal structures, an increasingly complete and coherent picture of polymerase structure and function is emerging. Recently determined structures of DNA and RNA polymerases have revealed some of the molecular features and structural changes governing catalysis, oligomerization, processivity and fidelity. Despite having minimal similarities in sequence and protein topology, the polymerases all display a functionally analogous set of subdomains that bind the primer, template and nucleotide substrates in similar though not identical fashions. The two-metal-ion mechanism for nucleotide incorporation, however, is shared even by nonhomologous polymerases.