Over the last decade, advances in subunit vaccine technology, achieved in many cases with recombinant DNA techniques, have created a dramatic increase in demand for vaccine adjuvants which can help to ellicit protective responses from subunit antigens which, in general, are poorly immunogenic when administered in the absence of an adjuvant (1). The purpose of this chapter is to summarize our extensive experience with the oil-in-water emulsion adjuvant MF59 and to emphasize to the reader that MF59 is no longer a research formulation, but a functional commercial adjuvant. Here we provide information on good manufacturing processes (GMP), and methods of characterization for postproduction release and demonstration of long-term stability of MF59, as well as representative data to demonstrate the consistency of the product. We also provide information on the in vitro and in vivo performance of MF59 in combination with various vaccine antigens, which we have gathered during the decade of development, which has resulted in formulation of MF59 in the Fluad^® vaccine, that marks the first European approval of a nonalum adjuvant for human use.