Bacterial names are continually being changed in order to more adequately describe natural groups (the units of microbial diversity) and their relationships. The problems in Klebsiella taxonomy are illustrative and common to other bacterial genera. Like other bacteria, Klebsiella spp. were isolated long ago, when methods to identify and classify bacteria were limited. However, recently developed molecular approaches have led to taxonomical revisions in several cases or to sound proposals of novel species.