Vaccine-delivery systems are generally particulate (e.g., emulsions, microparticles and liposomes) and have comparable dimensions to the pathogens, which the immune system evolved to combat. Increasingly more sophisticated delivery systems are being developed in which immunostimulatory adjuvants may be incorporated with the antigen. The rationale for this approach is to ensure that both the antigen and adjuvant are delivered into the same population of antigen-presenting cells. Enhancement of adjuvant activity through the use of micro- and nanoparticulate delivery systems is particularly exciting, as synergistic effects are often seen resulting in immune responses stronger than those elicited by the adjuvant or delivery system alone. Micro- and nanoparticles also offer the possibility of enhancement of their uptake by appropriate cells through manipulation of their surface properties. The next important step in the development of many of the experimental microparticle- and nanoparticle-based technologies will be evaluation of effectiveness in human trials.