The One Health paradigm for global health recognizes that most new human infectious diseases will emerge from animal reservoirs. Little consideration has been given to the known and potential zoonotic infectious diseases of small companion animals. Cats and dogs closely share the domestic environment with humans and have the potential to act as sources and sentinels of a wide spectrum of zoonotic infections. This report highlights the lack of a coordinated global surveillance scheme that monitors disease in these species and makes a case for the necessity of developing a strategy to implement such surveillance. Increasingly, the concept of One Health is recognized as a valuable paradigm for global health management. One Health is an initiative that seeks greater integration of human and veterinary medicine in areas as diverse as infectious disease control and comparative and translational medical research. A major focus of One Health has been on infectious diseases shared by humans, production animals (e.g., cattle, sheep, pigs, and poultry), companion animals, and wildlife in the context of ecosystems and the physical environment. An area of One Health that has garnered much attention has been the emergence or reemergence of infectious disease, and the finding that ≈75% of newly reported human infections have emerged, and are therefore likely to continue to emerge, from an animal reservoir (1). Such zoonotic infections could be of the following types: infections transmitted directly from animals to humans; vector-borne infections in which an animal or human is infected by the vector; or infections in which animals act as a reservoir for disease transmission, including having the potential for contaminating human food and water sources. A key goal of the evolving One Health paradigm includes surveillance of infectious diseases in domestic and wild animals to anticipate emergence of new zoonoses and protect humans. To achieve this goal, it is essential that global resources be allocated for more effective disease surveillance and reporting schemes that incorporate environmental, human, and veterinary health professionals. Many systems are in place nationally or globally to monitor human and production animal (and to a lesser extent, wild animal) disease (2), but major gaps in surveillance remain, particularly the lack of a surveillance infrastructure that includes companion animals. From a One Health perspective, companion animals can serve as sources of zoonotic infections, as intermediate hosts between wildlife reservoirs and humans, or as sentinel or proxy species for emerging disease surveillance (3). The aims of this review are to define and quantify the role of companion animals in the human domestic and peridomestic environment, highlight the major companion animal zoonoses and the potential for emergence of new human infections transmitted from these species, emphasize the lack of global infectious disease surveillance in these species against the current background of human and production animal surveillance, and suggest how to address this major One Health deficiency in the future.