The alpha-proteobacterial genus Bartonella comprises numerous arthropod-borne pathogens that share a common host-restricted life-style, which is characterized by long-lasting intraerythrocytic infections in their specific mammalian reservoirs and transmission by blood-sucking arthropods. Infection of an incidental host (e.g. humans by a zoonotic species) may cause disease in the absence of intra-erythrocytic infection. The genome sequences of four Bartonella species are known, i.e. those of the human-specific pathogens Bartonella bacilliformis and Bartonella quintana, the feline-specific Bartonella henselae also causing incidental human infections, and the rat-specific species Bartonella tribocorum. The circular chromosomes of these bartonellae range in size from 1.44 Mb (encoding1,283 genes) to 2.62 Mb (encoding 2,136 genes). They share a mostly synthenic core genome of 959 genes that features characteristics of a host-integrated metabolism. The diverse accessory genomes highlight dynamic genome evolution at the species level, ranging from significant genome expansion in B. tribocorum due to gene duplication and lateral acquisition of prophages and genomic islands (such as type IV secretion systems that adopted prominent roles in host adaptation and specificity) to massive secondary genome reduction in B. quintana. Moreover, analysis of natural populations of B. henselae revealed genomic rearrangements, deletions and amplifications, evidencing marked genome dynamics at the strain level.