Are viruses more biologically successful than cellular life? Here we examine many ways of gauging biological success, including numerical abundance, environmental tolerance, type biodiversity, reproductive potential, and widespread impact on other organisms. We especially focus on successful ability to evolutionarily adapt in the face of environmental change. Viruses are often challenged by dynamic environments, such as host immune function and evolved resistance as well as abiotic fluctuations in temperature, moisture, and other stressors that reduce virion stability. Despite these challenges, our experimental evolution studies show that viruses can often readily adapt, and novel virus emergence in humans and other hosts is increasingly problematic. We additionally consider whether viruses are advantaged in evolvability-the capacity to evolve-and in avoidance of extinction. On the basis of these different ways of gauging biological success, we conclude that viruses are the most successful inhabitants of the biosphere.