To provide a review of clinically relevant observations related to Bartonella species as emerging pathogens in veterinary and human medicine. Literature as cited in PubMed and as generated by each of the authors who have contributed to various aspects of the clinical understanding of bartonellosis. Important historical and recent publications illustrating the evolving role of animal reservoirs as a source of human infection. Comprehensive review of the veterinary literature. In addition to inducing life-threatening illnesses, such as endocarditis, myocarditis, and meningoencephalitis and contributing to chronic debilitating disease, such as arthritis, osteomyelitis, and granulomatous inflammation in cats, dogs, and potentially other animal species; pets and wildlife species can serve as persistently infected reservoir hosts for the transmission of Bartonella spp. infection to veterinary professionals and others with direct animal contact.