Drugs are rarely, if ever, administered to patients in an unformulated state. The vast majority of the available medicinal compounds which are potent at the milligram or microgram levels could not be presented in a form providing an accurate and reproducible dosage unless mixed with a variety of excipients and converted by controlled technological processes into medicines. Indeed, the primary skills of the pharmacist lie in the design, production and evaluation of a wide range of dosage forms, each providing an optimized delivery of drug by the selected route of administration. The aims of this chapter, therefore, are to outline mechanisms by which the onset, duration and magnitude of the therapeutic responses can be controlled bv the designer of the drug delivery system.