This article by W. Ford Doolittle (Microbiology Today, Nov 2006) provides a brief overview of the debate regarding the definition of a species when dealing with prokaryotes. He outlines the advantages and disadvantages of using DNA sequence data to define species and discusses molecular processes, such as homologous recombination and lateral gene transfer, that generate genetic diversity in prokaryotes. There is also a brief list of suggested additional reading. This would be appropriate reading for advanced undergraduate or graduate students.