Evolutionary biology is not widely regarded as a predictive science. However, accurate predictions of evolvability would be particularly useful in the evolution of infectious diseases, such as the ability to preemptively address the challenge of pathogens newly emerging in humans and other host populations. Experimental evolution of viruses offers the possibility to rigorously test hypotheses regarding pathogen evolvability. This seminar provides examples from various model systems used to study virus emergence, especially RNA phage that infect bacteria, as well as arthropod-borne RNA viruses that attack eukaryotes.