Bacteria have evolved efficient mechanisms to sense environmental changes to which they respond with high sensitivity and specificity. Reprogramming gene transcription by directing RNA polymerase to specific promoters through alternative σ subunits is a wide-spread bacterial strategy of rapid adaptation to environmental challenges. Extracytoplasmic function (ECF) σ factors represent the most minimalistic and diverse group of the σ70 protein family. These σ factors are typically stress-inducible and non-essential. Most bacterial species possess multiple ECF σ factors. Here, we review the current knowledge on promoter specificity, regulatory control and biological function of ECF σ factors in members of the α-proteobacterial Rhizobiales, which enter a root nodule symbiosis with leguminous host plants.