Current approaches to the development of drug delivery systems are based typically on the premise that the relationship between the plasma concentration and therapeutic effect of a drug is invariant with time. Release kinetics from these systems are, therefore, usually engineered to be either zero-order or a simple function of time. It has been recognised for some time, however, that this approach may not be appropriate for certain drugs and it has been suggested that therapeutic efficacy may be improved by the utilisation of triggered, pulsed and programmed delivery systems. This article considers an approach to the development of responsive drug delivery system based on the utilisation of functional polymers, which aim to improve therapeutic efficacy by varying drug release in accordance with a patients's varying requirements.