Csr (carbon storage regulator) is a recently discovered global regulatory system that controls bacterial gene expression post-transcriptionally. Its effector is a small RNA-binding protein referred to as CsrA or, in phytopathogenic Erwinia species, RsmA (repressor of stationary phase metabolites). Numerous genes whose expression occurs in the stationary phase of growth are repressed by csrA/rsmA, and csrA activates certain exponential-phase metabolic pathways. Glycogen synthesis and catabolism, gluconeogenesis, glycolysis, motility, cell surface properties and adherence are modulated by csrA in Escherichia coli, while the production of several secreted virulence factors, the plant hypersensitive response elicitor HrpN(Ecc) and, potentially, other secondary metabolites are regulated by rsmA in Erwinia carotovora. CsrA represses glycogen synthesis by binding to and destabilizing glgCAP mRNA and is hypothesized to repress other genes by a similar mechanism. The second component of the Csr system is CsrB (AepH in Erwinia species), a non-coding RNA molecule that forms a large globular ribonucleoprotein complex with approximately 18 CsrA subunits and antagonizes the effects of CsrA in vivo. Highly repeated sequence elements found within the loops of predicted stem-loops and other single-stranded segments of CsrB RNA may facilitate CsrA binding. Current information supports a model in which CsrA exists in an equilibrium between CsrB and CsrA-regulated mRNAs, which predicts that CsrB levels may be a key determinant of CsrA activity in the cell. The presence of csrA homologues in phylogenetically diverse species further suggests that this novel kind of regulatory system is likely to play a broad role in modulating eubacterial gene expression.