The use of adjuvants for immunopotentiation has been investigated since the 1920s and a number of comprehensive reviews and monographs have been published on this subject. A recent trend in immunopotentiation has been the use of delivery systems which allow for sustained or controlled release of antigens and which induce prolonged immunity following a single dose. This concept has been termed either single-step or single-shot immunization. The delivery system has been modulated to potentiate the immune response either by delivering the antigen (and perhaps an adjuvant or adjuvants) either over a prolonged period of time or in a predetermined sequence or by incorporating substances with immunoadjuvant properties (e.g., lecithin and certain biodegradable polymers) as carriers within the delivery system. This Review focuses on the progress made in the design of delivery systems for immunopotentiation. Particular emphasis is given to delivery systems designed to achieve single-step immunization.