Advancements in the understanding of robustness and its evolution have often arrived through theoretical studies, [5] [6] [7] [8] but empirical studies have made recent in-roads. Experiments using artificial life ͑"digital organisms," selfreplicating computer programs that can evolve͒ valuably demonstrated that elevated mutation rates can select for evolved increases in genetic robustness to tolerate mutation, even at the expense of reduced reproductive fitness. 9 The explanation was that high mutation rates could selectively favor genetic variants that were not necessarily productive and resided on flat regions of the "fitness landscape;" these robust genotypes formed an epistatic network that produced equally fit phenotypes despite mutation-induced movement across the landscape ͑Fig. 1͒. 6, 8, 10 Other landmark studies have successfully examined robustness by considering phenotypic effects of mutations underlying proteins using computational and in vitro approaches. 11, 12 Viruses with RNA genomes are natural systems that typically experience high mutation rates owing to their lack of error-repair during replication. Thus, RNA viruses have proved to be useful and tractable models for studying robustness evolution in biological populations. This work has focused on the success of robust versus nonrobust RNA virus variants when mutation rates are further elevated through exposure to ultraviolet ͑UV͒ light and other mutagens, 13 and on evolved changes in robustness under frequent virus coin-fection, which allows buffering of mutational effects via complementation. 14 Below we review some of the evidence from these studies, and present new findings from an increasingly popular model system for examining evolution of robustness: the segmented RNA bacteriophage 6. [14] [15] [16] In reviewing these results, we hope to highlight the importance of empirical work in RNA viruses for testing theory pertaining to robustness, as well as for better understanding the evolutionary biology and evolvability of infectious organisms in general.