The maturation and degradation of RNA molecules are essential features of the mechanism of gene expression, and provide the two main points for post-transcriptional regulation. Cells employ a functionally diverse array of nucleases to carry out RNA maturation and turnover. Viruses also employ cellular ribonucleases, or even use their own in their reproductive cycles. Studies on bacterial ribonucleases, and in particular those from Escherichia coli, are providing insight into ribonuclease structure, mechanism, and regulation. Ongoing biochemical and genetic analyses are revealing that many ribonucleases are phylogenetically conserved, and exhibit overlapping functional roles and perhaps common catalytic mechanisms. This article reviews the salient features of bacterial ribonucleases, with a focus on those of E. coli, and in particular, ribonuclease III. RNase III participates in a number of RNA maturation and RNA decay pathways, and is regulated by phosphorylation in the T7 phage-infected cell. Plasmid and phage RNAs, in addition to cellular transcripts, are RNase III targets. RNase III orthologues occur in eukaryotic cells, and play key functional roles. As such, RNase III provides an important model with which to understand mechanisms of RNA maturation, RNA decay, and gene regulation.