Recently, the Escherichia coli umuD and umuC genes have been shown to encode E. coli's fifth DNA polymerase, pol V (consisting of a heterotrimer of UmuD'(2)C). The main function of pol V appears to be the bypass of DNA lesions that would otherwise block replication by pols I-IV. This process is error-prone and leads to a striking increase in mutations at sites of DNA damage. While the enzymatic properties of pol V are now only beginning to be fully appreciated, a great deal is known about how E. coli regulates the intracellular levels of the Umu proteins so that the lesion-bypassing activity of pol V is available to help cells survive the deleterious consequences of DNA damage, yet keeps any unwarranted activity on undamaged templates to a minimum. Our review summarizes the multiple restrictions imposed upon pol V, so as to limit its activity in vivo and, in particular, highlights the pivotal role that the N-terminal tail of UmuD plays in regulating SOS mutagenesis.