The formation of spores in Bacillus subtilis is a developmental process under genetic control. The decision to either divide or sporulate is regulated by the state of phosphorylation of the SpoOA transcription factor. Phosphorylated SpoOA (SpoOA approximately P) is both a repressor and an activator of transcription depending on the promoter it is affecting. SpoOA approximately P is the end product of the phosphorelay, a signal transduction system linking environmental information to the activation of sporulation. Activation or deinhibition of two ATP-dependent kinases, KinA and KinB, to phosphorylate the SpoOF secondary messenger initiates the phosphorelay. SpoOF approximately P is the substrate for the SpoOB protein, a phosphoprotein phosphotransferase which transfers the phosphate group to SpoOA. The SpoOA approximately P formed from this pathway orchestrates transcription events during the initial stage of spore development through direct effects on a variety of promoters and through the use of other transcription factors, termed transition state regulators, whose activity it controls. Because commitment to sporulation has serious cellular programming consequences and is not undertaken capriciously, the phosphorelay is subject to a variety of complex controls on the flow of phosphate through its components.