Adjuvants are essential components of most clinically used vaccines. This is because the majority of nonliving vaccines are relatively poor inducers of adaptive immunity unless effective adjuvants are co-administered. Aluminum salts (alum) have been used as adjuvants with great success for almost a century and have been particularly effective at promoting protective humoral immunity. However, alum is not optimally effective for diseases where cell-mediated immunity is required for protection. Furthermore, adjuvants including oil-in-water emulsions have shown improved efficacy for avian influenza protection suggesting that even for diseases where humoral immunity can confer protection, there is scope for developing improved adjuvants. There have been major developments in antigen discovery over the past decade, which has accelerated the vaccine development process for new indications and this demands a new generation of adjuvants that can drive and specifically direct the desired immune responses. A number of systems are under investigation that combine different types of adjuvants into specific formulations with greater activity. Additionally, targeting of vaccines to specific immune cells shows great promise. In the case of cancer and chronic infectious diseases, it may be difficult to develop effective vaccines without blocking immune regulatory pathways, which impede cell-mediated responses. However, increased understanding of immunology and particularly the innate immune system is informing vaccine adjuvant research and consequently driving the development of novel and specifically directed vaccine adjuvant strategies. In this article we address the importance of adjuvants in vaccine development, the known mode of action of specific adjuvants and recent developments in this important field.