The relative impermeability of the cytoplasmic membrane to protons, and at least some other cations, may critically relate to the energization of a variety of processes, including ATP synthesis, many solute transport systems, and bacterial motility. According to the chemiosmotic hypothesis of Mitchell, a protonmotive force (PMF) is generated by the extrusion of protons during electron transport or during ATP hydrolysis. This chapter reviews reports on the magnitude of the PMF and factors involved in its establishment. It discusses some of the controversies surrounding these determinations. Among the processes in bacterial cells that are believed to be energized by the PMF are ATP synthesis, energy-linked transhydrogenase, many active transport systems, photosynthesis, and motility. In view of the many bioenergetic functions in bacteria, a plethora of specialized membranous structures, adapted in particular ways, might have been expected to exist.