Recent years have witnessed a dramatic global growth in the development of large-scale public science events. Although usually grouped together under the umbrella term ‘Science Festivals’, the events differ greatly in size and scope. This paper presents the findings from a 2008/09 international survey of Science Festivals in order to compare and contrast worldwide trends. An online survey was completed by 56 self-identified Science Festivals, supplemented by a content analysis of 94 Festival websites identified internationally. This work identifies for the first time a common international definition for events which identify themselves as ‘Science Festivals’. The findings show that Science Festivals are currently particularly common within Europe; however, their popularity is growing within other regions. There is a large diversity in the scale of Science Festivals, encompassing some small, localised events reaching a few hundred people, up to nationwide events reaching many millions. Precise audience figures are not acquired by many Festivals; however, there is evidence that over 5.6 million people are reached by Science Festivals annually, with events focused mainly on hands-on activities combined with some talks, lectures, discussions, and debates. The funding and operational modes also vary significantly, with the vast majority of Science Festivals obtaining their funding from multiple sources, including government support, sponsorship, and funding grants. A considerable number of Science Festivals conduct at least an informal evaluation, with some Festivals making their evaluations publicly available. This work demonstrates that Science Festivals are an increasingly important area of science communication worthy of further research.