Bacterial signal transduction involves one- and two-component systems and extracytoplasmic function σ factors. With its simple design, the one-component system is the most widely distributed type of bacterial signal transduction system. The design of two-component systems simplifies transmembrane signaling and also enables many modifications, including amplification and integration of different signals. The activity of ECF σ-factors is regulated by different mechanisms through a diverse set of anti- σ molecules. Although the two-component system and ECF σ factors are phylogenetically and biochemically unrelated, they share several mechanistic analogies while differing in the way their respective two-protein components communicate with each other.