RNA viruses face dynamic environments and are masters at adaptation. During their short "lifespans," they must surmount multiple physical, anatomical and immunological challenges. Central to their adaptive capacity is the enormous genetic diversity that characterizes RNA virus populations. Although genetic diversity increases the rate of adaptive evolution, low replication fidelity can present a risk because excess mutations can lead to population extinction. We have used the poliovirus model system to study how mutational robustness might influence viral evolution and pathogenesis. Newer work that extends these findings to influenza virus will also be discussed.