In bacteria, sigma factors are essential for the promoter DNA-binding specificity of RNA polymerase. The sigma factors themselves are regulated by anti-sigma factors that bind and inhibit their cognate sigma factor, and 'appropriators' that deploy a particular sigma-associated RNA polymerase to a specific promoter class. Adding to the complexity is the regulation of anti-sigma factors by both anti-anti-sigma factors, which turn on sigma factor activity, and co-anti-sigma factors that act in concert with their partner anti-sigma factor to inhibit or redirect sigma activity. While sigma factor structure and function are highly conserved, recent results highlight the diversity of structures and mechanisms that bacteria use to regulate sigma factor activity, reflecting the diversity of environmental cues that the bacterial transcription system has evolved to respond.