There is an increasing trend away from classical attenuated or killed whole pathogen vaccines towards developing chemically defined preparations. Paradoxically, in order to be effective, these defined vaccines require incorporation into adjuvants, of which very little is actually understood about how or why they work. It is, therefore, of little surprise that, in this context, adjuvants have been referred to as 'the immunologists' dirty little secret'. [Janeway CA, Approaching the asymptote? Evolution and revolution in immunology. Cold Spring Harbor Symp Quantit Biol (1989) 54: 1-13]. However, modern techniques in immunology have made available a number of powerful tools that will allow a more complete dissection of how traditional adjuvants work, knowledge of which will not only facilitate the rational design of new adjuvants but also further clarify our understanding of the immune system. The initial studies described in this review indicate that cytokines play an important, if not the central, role in the ability of adjuvants to influence both the quantity and quality of immune responses.