The monomeric bacteriophage RNA polymerases allow the synthesis of virtually any RNA molecule in unlimited quantity. In this protocol, we describe the preparation of plasmid and PCR-derived templates. A basic transcription protocol is provided with several optional modifications. The use of RNA probes in Northern blot hybridization and in RNase protection assays is described. The relative advantages and pitfalls of these two methods to quantitatively detect mRNA targets are discussed.