The basic task of microbial taxonomy is to differentiate microorganisms. In the 1870s, Ferdinand Cohn first proposed bacterial subdivisions, albeit based on morphological criteria only. (cf. Leopoldina Symposium “Bacterial Pathogenesis – Modern Approaches”, 1999). Since then, taxonomy has made substantial progress: from the first edition of Bergey`s Manual of Determinative Bacteriology in 1923 (which used morphological and biochemical markers) to the present-day molecular based methods such as DNA-DNA hybridization, 16S rRNA sequencing, and the use of polyphasic taxonomy and of typing systems. This symposium will highlight some of the applications of these methods including genomics and proteomics and their use for a variety of purposes, e.g., genus and species delineation, analysis of as yet unculturable bacteria, and communities of microorganisms. It will also present data on viral and parasitic taxonomy. A complete review of the field is not intended. Emphasis will also be on possible future developments.