The need for potentiating immune responses to recombinant or subunit antigens has prompted GlaxoSmithKline (GSK) Biologicals to develop various Adjuvant Systems for the design of prophylactic and therapeutic vaccines. Adjuvant Systems are formulations of classical adjuvants mixed with immunomodulators, specifically adapted to the antigen and the target population. They can activate the appropriate innate immune system and subsequently impact on adaptive immune responses. AS04 is an Adjuvant System that has demonstrated significant achievements in several vaccines against viral diseases. AS02, another Adjuvant System, is being evaluated in various contexts, where a strong T-cell response is needed to afford protection. Likewise, AS01 has been developed for vaccines where the induction of a yet stronger T-cell-mediated immune response is required. Altogether, the promising clinical results strongly support the concept of Adjuvant Systems and allow for further development of new vaccines, best adapted to the target population and the immune mechanisms of protection.