Adjuvants can be broadly divided into two groups, based on their principal mechanisms of action: vaccine delivery systems and immunostimulatory adjuvants. Vaccine delivery systems are generally particulate (e.g., emulsions, microparticles, immunostimulatory complexes and liposomes) and function mainly to target associated antigens into antigen-presenting cells. However, increasingly, more complex formulations are being developed in which delivery systems are exploited both for the delivery of antigens and also for the delivery of coadministered immunostimulatory adjuvants. The rationale for this approach is to ensure that both antigen and adjuvant are delivered into the same population of antigen-presenting cells. In addition, delivery systems can focus the effect of the adjuvants onto the key cells of the immune system and limit the systemic distribution of the adjuvant, to minimize its potential to induce adverse effects. The formulation and delivery of potent adjuvants in microparticles may allow the development of prophylactic and therapeutic vaccines against cancers and chronic infectious diseases, which are currently poorly controlled. In addition, microparticle formulations may also allow vaccines to be delivered mucosally.