The chapter gives a survey of the different human pathologies caused by Bacillus spp. and mainly those of the B. cereus group. Before giving details on the clinical features, toxicology and the methods of detection of these bacteria and their corresponding pathogenecity factors, the complex ecology and the identification challenges that characterize several Bacillus species is briefly introduced. The B. cereus group that shows a ‘bivalent face’ as far as its important impact on human activity was used as a paradigm.