Microbial growth and metabolism play key roles in all of the major geochemical cycles of Planet Earth, consuming and replenishing carbon, nitrogen, hydrogen and sulphur in the biosphere. Microbes can be extremely beneficial. They are essential tools in environmental biotechnology, biodegradation, composting, the supply of fresh drinking water, the safe disposal of sewage, and in food, wine and beer production. But they can also cause disease and food deterioration. Microbial physiology is the study of microbial structure and function, in particular, how microbial activity responds to changes in the environment. It is therefore the key to controlling detrimental aspects of microbes, and to exploiting them for the good of the human race.