Bacteria vary tremendously in their cell structure and metabolism. They include heterotrophs, phototrophs, and lithotrophs, with some species that could be classified as all three. There are obligate aerobes, anaerobes, and microaerophiles. Their cell shapes include rods, cocci, spirals, and budding forms. Ecologically, bacteria include mutualists, pathogens, and organisms that cannot be cultured in our laboratories. New species, even entire new phyla of bacteria, are continually discovered in soil and water, in our homes, and even within our own bodies. The species we know represent but a tiny fraction of the diverse bacterial species growing in nature. This chapter describes some of the better known bacterial phyla.