Basic principles underlying the population dynamics of bacteria and viruses are presented, with emphasis on RNA viruses. Concepts reviewed here include fitness, mutant generation, competition, selection, sequence space and the theoretical origins of quasispecies. A "wild-type" virus is no longer viewed as the fittest type, but as the center of gravity of a population landscape. A consequence of quasispecies is the existence of an error threshold for selective competence. The error threshold has a justification in quasispecies theory and lends itself to exploration of strategies to drive viral populations over the error threshold, the central theme of this volume.