Viruses, the most abundant biological entities on Earth, play important roles in biogeochemical cycling, horizontal gene transfer, and defining bacterial community composition. Despite their abundance and importance, a number of fundamental questions still remain regarding the diversity, biogeography, and ecological roles of aquatic viruses. PCR-based studies reveal extremely high levels of diversity in individual samples, even within restricted viral phylogenetic groups. Combined with the frequent discovery of novel virus types (such as single-stranded DNA phages and marine circo-like viruses) and the fact that viruses infecting many important groups of marine animals have not yet been described, this suggests that we still have a long way to go before understanding the extent of marine viral diversity. In contrast, metagenomic studies suggest significant overlap in viruses found throughout the world’s oceans, and some viral genes have extremely wide biogeographical distributions. This talk will detail a time-series analysis of viral abundance and diversity at the Bermuda Atlantic Time-series Study Site and discuss current views of the extent of global viral diversity, with particular attention to newly described groups of single-stranded DNA viruses.