The Rickettsiales are an order of small proteobacteria. Most of those described survive only as endosymbionts of other cells. Some are notable pathogens, including Rickettsia, which causes a variety of diseases in humans. On the other end of the scale, genetic studies support the origin of mitochondria and related organelles developed from members of this group. Some have also speculated that viruses might have developed from them, or from organisms like them.

Environmental samples have found a variety of genes that originate from this group, pointing to a number of free-living planktonic species. These include SAR11, later cultivated as Pelagibacter ubique, one of the most common bacteria in the world.