This chapter is concerned with the identification, characterization, and behavior of aluminum-containing adjuvants with proteins and anions similar to those occurring in vaccines and interstitial fluid. Aluminum-containing adjuvants referred to commercially as aluminum hydroxide have been identified as poorly crystalline aluminum oxyhydroxide with the structure of the mineral boehmite. Relevant properties of this material include its high surface area and its high pI, which provide the adjuvant with a high adsorptive capacity for positively charged proteins. Aluminum phosphate and alum-precipitated adjuvants may be classified as amorphous aluminum hydroxyphosphate with little or no specifically adsorbed sulfate. Variations in the molar PO4/A1 ratio of amorphous aluminum hydroxyphosphates result in PI values that range from 5 up to 7; the materials are negatively charged at a physiological pH of 7.4. The amorphous nature of these compounds gives them high surface area and high protein adsorptive capacity for positively charged proteins. Observations on the interactions of anions and charged proteins with charged adjuvant surfaces have provided a framework for predicting behavior of complex systems of vaccines and for designing specific combinations of adjuvants and antigens to optimize the stability and efficacy of vaccines.