Adjuvants are agents or drugs that have few or no antigenic effects or properties but may have the potential to increase the efficacy or potency of vaccines or other drugs when given at the same time (see Chapter 9). Some of the most common adjuvants used in vaccine preparations are aluminum salts and monophosphoryl lipid (MPL)/QS21 cocktails. The response to potential pandemic flu vaccines has been shown to be much improved by the addition of MF59 (Nicholson et al., 2001; Stephenson et al., 2003; Atmar et al., 2006; Bresson et al., 2006; Baldo et al., 2007).