Adaptive designs have typically been used to increase the sample size after performing an interim analysis in confirmatory clinical trials of antiviral drugs. Adaptive designs have also been used in exploratory trials to select an optimal dose and dosage regimen, primary efficacy endpoint and targeted subgroups for further evaluation based on a comparison of the short-term antiviral activity, safety and tolerability. A dialogue between the sponsor and the FDA regarding prospective adaptive designs can begin as early as an End of Phase IIa meeting or during the planning of pivotal trials. Adaptive design considerations used to plan further clinical trials of antiviral drugs will be discussed. Examples of adaptive designs will involve the use of conditional power calculations for sample size re-estimation.