Bacteriophage T7 RNA polymerase (RNAP) is the best characterized member of a widespread family of RNAPs that includes most bacteriophage-encoded RNAPs as well as the mitochondrial RNAPs. The robust activity and strict promoter specificity of the phage RNAPs have made them extremely useful as reagents for both in vitro preparation of specific RNAs and for in vivo gene expression. The structural simplicity of these enzymes, relative to the larger multisubunit cellular RNAPs, has also made them attractive targets for structural, structure-function, and mechanistic studies. X-Ray crystal structures of T7 RNAP alone, in complexes with promoter and with a transcriptional regulator, and in a transcription complex have been determined. The structures reveal that despite extremely limited sequence similarity, the three-dimensional structure of T7 RNAP is very similar to that of other polymerases with different template and substrate specificities. Extensive structure-function studies have mapped specific T7 RNAP functions to defined regions of the polymerase, and even to individual amino acids. Studies of the transcription reaction mediated by this enzyme reveal a remarkably coordinated and conformationally dynamic process, with changes in polymerase conformation, DNA and RNA secondary structure, and nucleic acid:protein interactions occurring throughout the reaction. A sophisticated regulatory mechanism takes advantage of these dynamic processes to specifically control T7 RNAP activity so as to generate required proteins at appropriate times and amounts during the phage life-cycle.