Endemic and enzootic zoonoses cause about a billion cases of illness in people and millions of deaths every year, and emerging zoonoses are a rising threat to global health. Zoonoses – pathogenic organisms such as bacteria or viruses which we share with animals – cause more than 60% of human infectious diseases, and have been responsible for some of the most devastating disease outbreaks in recent years, including HIV, Ebola, and SARS. However, despite their huge, and rising, impact on human health, there are still huge gaps in our understanding of how zoonoses spread and develop, which need to be urgently addressed if we are to be able to reduce the impact of the next zoonotic pandemic. In a new Lancet Series, leading experts discuss the ecology, drivers and dynamics of zoonoses, while also addressing how we might predict the next zoonotic pandemic, and reduce the potentially catastrophic human and economic cost of such an outbreak. The Lancet Global Health Series are groups of articles on a topic of global health importance. Series papers and Comments, related articles, links to relevant publications, media highlights, and audio interviews are available from http://www.thelancet.com/global-health-series. Research into prevalent conditions and wider health policy issues are reported by a range of experts.