The structures of T7 RNA polymerase (T7 RNAP) captured in the initiation and elongation phases of transcription, that of phi29 DNA polymerase bound to a primer protein and those of the multisubunit RNAPs bound to initiating factors provide insights into how these proteins can initiate RNA synthesis and synthesize 6-10 nucleotides while remaining bound to the site of initiation. Structural insight into the translocation of the product transcript and the separation of the downstream duplex DNA is provided by the structures of the four states of nucleotide incorporation. Single molecule and biochemical studies show a distribution of primer terminus positions that is altered by the binding of NTP and PP(i) ligands. This article reviews the insights that imaging the structure of polynucleotide polymerases at different steps of the polymerization reaction has provided on the mechanisms of the polymerization reaction. Movies are shown that allow the direct visualization of the conformational changes that the polymerases undergo during the different steps of polymerization.