Specific and tight control of gene expression is an essential prerequisite for a cell to survive in changing habitats. In bacteria, alternative σ factors are an important regulatory principle to control the expression of a defined set of genes in response to environmental cues or in the course of cellular differentiation programs. The extracytoplasmic function (ECF) σ factors represent the most diverse and abundant group of alternative σ factors that typically play a role in the bacterial stress response. The diversity of ECF σ factors is reflected in the mechanisms by which their activity is regulated, their protein architecture, and their target promoter sequence. Five years ago, we proposed a classification of this protein family, based on sequence similarity and domain architecture of both the ECF σ factor and its cognate anti-σ factor, genomic context conservation, and putative target promoter motifs. This classification not only reflects the phylogeny of ECF σ factors but also allows proposing detailed hypotheses on potential novel ECF-dependent regulatory mechanisms that can be tested experimentally. Initially, we identified 66 groups; since then, follow-up studies have identified 28 additional groups. In this chapter, we provide a comprehensive and up-to-date compilation of the currently available information on these 94 defined ECF groups. It was written with the intention of serving as a concise desk reference for those interested in the regulatory diversity of this third pillar of bacterial signal transduction.