Most antiviral agents have been discovered by chance. These discoveries and the rationalization of their mode of action in terms of their selective action in the viral replicative cycle has demonstrated that viral targets exist. It is unlikely that in the near future that rational design will result in the synthesis of new classes of antiviral agents. Progress is more likely from a combination of the continuing identification of potential targets and the design of screens to identify prototype compounds, the properties of which may then be improved by chemical synthesis of analogues.