 Much of the debate over species pluralism focuses on multicellular organisms. In other words, the debate focuses on how to sort mammals, birds, fish, plants, and other multicellular organisms into species. However, most of life is not multicellular. Most organisms in the world are single cell microbial organisms (Rossello-Mora and Amann 2001). Just as biologists sort multicellular organisms into species, microbiologists sort microbial organisms into species. Advancements in genetic technology in the last thirty years have caused an increase in taxonomic work in microbiology, including ways to think about microbial species. Microbial biologists have their own species concepts—definitions of ‘species’ that apply to only microbial organisms. Microbial species concepts vary, and as we shall see, cross-classify microbial organisms into different species. Once again we come to the question of species pluralism.