We generally view viruses as agents of disease and death. Yet, in reality we rely on natural viral populations for our very survival. Over the last 10 years our view of the role of viruses in aquatic environments has changed dramatically, to the same extent as the paradigm shift of the critical role of bacteria in the 80's. Rapid probe methods have shown that the abundance of phage (viruses that infect bacteria) is often 10 times the number of bacteria and a large proportion of these bacteria are themselves infected. Phage are now seen to control bacterial diversity and ecosystem function with viral replication dependent on the bacterial host metabolic state and growth rate. Phage may be lytic (infect and immediately lyse the host) or lysogenic where infection leads to a mutualistic non-lytic host/viral relationship. The latter can be disrupted by physical and chemical changes to the host's aquatic environment. Viral lysis of bacteria releases dissolved organic carbon (DOC) that can influence biogeochemical cycles, particularly the carbon cycle.