The genus Brucella belongs to the class Alphaproteobacteria which is one of the largest and diverse groups within the phylum Proteobacteria. Comparative genome analysis revealed that Brucella is genetically related to plant-associated symbionts and pathogens. The genus Ochrobactrum, consisting of saprophytes that occasionally infect humans, is the closest phylogenetic neighbour. For many years, the taxonomy of Brucella remained unchanged. The recent development of new molecular typing methods and comparative genome analysis however, has resulted in rapid advances in the understanding of Brucella diversity and, after many years of stagnancy, a process of expansion of the genus is in progress. The recent increased occurrence of novel atypical Brucella strains and novel species poses a new challenge to the characterization, classification and nomenclature of the genus Brucella. Genome sequencing projects of these new isolates have been initiated to obtain a better understanding of biology (including pathogenicity) and evolution of members of the genus Brucella.