The ultimate goal is to achieve a theory-based classification system based on a phylogenetic/evolutionary concept. However, there are currently two contradictory opinions about the future classification of Bacteria and Archaea. Agroupof mostly molecular biologists posits that the yet-unclear effect of gene flow, in particular lateral gene transfer, makes the line of descent difficult, if not impossible, to describe. However, even in the face of genomic fluidity it seems that the typical geno-and phenotypic characteristics of a taxon are still maintained, and are sufficient for reliable classification and identification of Bacteria and Archaea. There are many well-defined genotypic clusters that are congruent with known species delineated by polyphasic approaches. Comparative sequence analysis of certain core genes, including rRNA genes, may be useful for the characterization of higher taxa, whereas various character genes may be suitable as phylogenetic markers for the delineation of lower taxa. Nevertheless, there may still be a few organisms which escape a reliable classification. & 2009 Elsevier GmbH. All rights reserved.