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At abundances routinely greater than 10 million particles per milliliter, viruses are the most numerous biological entities in the oceans. To put the sheer abundance of marine viruses in context, we note that they contain more carbon than 75 million blue whales and, if such viruses were joined end-to-end, they would stretch further than the nearest 60 galaxies (Suttle, 2005). While marine viruses were first described by Spencer (1955), they were largely ignored for three decades because of the relatively low abundances inferred using culture-based assays. However, since Bergh et al. (1989) recognized their numeric importance, they have been considered at least as abundant as marine microbes, and scientists have been characterizing them and trying to determine the extent of marine viral diversity. Extensive efforts have focused on understanding the role of viruses in horizontal gene transfer and microbial mortality, and on the consequent impacts on microbial abundance, diversity, and community structure. Here, we review advances in understanding viral diversity and genome evolution, and discuss potentially fruitful areas for future research. Our emerging view of the virosphere, inferred from gigabases of sequence data ground truthed by model systems in culture, is one of immense but finely tuned genetic diversity, where viruses have seemingly endless genetic potential, yet clearly are maintaining key genetic elements to propagate their extraordinary success.