This chapter reviews the bacteriophage N4 vRNAP and its unique ability to transcribe promoter-containing single-stranded DNAs with specificity, providing a tool for the synthesis of RNAs from synthetic oligonucleotide templates or from templates cloned in M13 DNA. It is an ideal enzyme for the synthesis of short RNAs in large amounts at limiting template concentrations in the presence of Eco SSB, for several reasons. N4 vRNAP displays high fidelity and, in contrast to T7 RNAP, does not appear to produce abortive products. It discusses the length of the transcripts that can be synthesized by mini-vRNAP is limited by the length of the available single-stranded DNA template. vRNAP is useful for the synthesis of specific RNA–DNA hybrids when transcription is carried out in the absence of Eco SSB. The chapter explores the discovery of mini-vRNAP that has provided a useful tool to study polymerase–promoter, polymerase–substrate, and polymerase–product interactions. Finally, identification of the vRNAP transcriptionally active domain raises questions on the possible roles of the amino and carboxy-terminal domains of the polypeptide. Experiments in progress have indicated roles for these domains in virion morphogenesis, and the initial steps of phage infection.