The genus Bacillus comprises a heterogeneous group of bacteria that have long been united into one genus because of their gram positive cell wall and their capacity to form endospores. The classification of this group of bacteria have changed importantly because of new developments in bacterial taxonomy and classification. Indeed the introduction of comparative sequence analysis of the 16S rDNA has changed the bacterial taxonomic picture. This 16S rDNA sequence alone does not allow to discriminate at the species and subspecies level. To reach these fine taxonomic levels a polyphasic approach based on a variety of techniques, both genotypic and genotypic supports a consensus classification. Due to these new approaches, the genus Bacillus has been rearranged taxonomically and consists now of about 15 genera. The discovery of new species and genera within this very diverse group of gram positive bacteria that belong to the low 0+C rDNA division, is in full development. It becomes more and more clear that representatives of Bacillus and related taxa are ubiquitous and can survive extreme conditions that are (e.g. sterilisation processes) or are not (e.g. extreme climate conditions found on e.g. Antarctica) related to human activities. In this context these organisms play an important role in the mineralisation processes in nature and also as contaminants in e.g. food processes. Furthermore member of the genus Bacillus and related organisms are important as active component in bacterial processes (production of antibiotica, different industrial enzymes etc). They are also used as bioinsecticides against a variety of insects whether or not integrated in the Eukaryotic geonomes. Some of them are important pathogens for men and animal. Finally, their evntual contribution to stimulate plantgrowth and their potentialities in bioremediation processes in currently under investigation. In relation to these aspects, the biodiversity of the bacilli is studied in different national and international projects.