Bacterial adaptation to suboptimal nutrient environments, including host and/or extreme environments, is subject to complex, coordinated control involving many proteins and RNAs. Among the γ-proteobacteria, which includes many pathogens, the RpoS regulon has been a key focus for many years. Although the RpoS regulator was first identified as a growth phase-dependent regulator, our current understanding of RpoS is now more nuanced as this central regulator also has roles in exponential phase, biofilm development, bacterial virulence and bacterial persistence, as well as in stress adaptation. Induction of RpoS can also exert substantial metabolic effects by negatively regulating key systems including flagella biosynthesis, cryptic phage gene expression and the tricarboxylic acid cycle. Although core RpoS-controlled metabolic functions are conserved, there are substantial differences in RpoS regulation even among closely related bacteria, indicating that regulatory plasticity may be an important aspect of RpoS regulation, which is important in evolutionary adaptation to specialized environments.