A cutting-edge review of the major research areas of adjuvant discovery, design, development, and use. The authors lay down a rational basis for vaccine adjuvant function and analyze a number of significantly distinct adjuvant-active molecules to illuminate the principles of their function and use. The focus is on specific receptor-ligand interactions, including the molecular features needed for a compound to possess adjuvant activity. The critical interface zone between the innate and adaptive immune systems is also analyzed to show how adjuvants exert their effects on T- and B-cell activation. Additional chapters address the possibility of tailoring adjuvants to yield optimally safe and effective responses.