Successful vaccination against infectious or neoplastic disease programs the host's immune system, in a multistep process, to generate an efficient defense and memory response. Conditioning of immune responses to nonreplicating, poorly immunogenic antigens generally requires the help of an adjuvant. The present review attempts to classify vaccine adjuvants functionally, according to recently proposed, mutually exclusive concepts of immunity induction. These include the geographical concept of immune reactivity and the theory of depot effect. Both emphasize the importance of antigen delivery and localization to the lymph node in time after immunization. Other concepts stress the importance of key signals, such as" infectious nonself" or" danger," which influence the activation state of the antigen-presenting cell (APC) and, hence, its capacity to prime naive T cells. The nature of adjuvant-induced immune responses is discussed in relation to each concept.