Manufacturers that use diploid strains or continuous cell lines to produce vaccines or biopharmaceuticals usually generate, characterize, and maintain their own cell banks. This chapter focuses on the use of cultured cells in biopharmaceutical manufacturing, the central role of cell banks in process standardization, and the special strategies and conditions which must be employed in the production and maintenance of these cell banks. It provides a brief description of the characterization of cell banks. One of the most important advantages of using serially cultured cells (diploid cell strains or continuous cell lines) to produce vaccines or biopharmaceuticals is the ability to have a characterized common starting source of product for each production lot, i.e., the cryopreserved master cell bank (MCB). Exhaustive characterization of the MCB allows standardization of the starting source for all subsequent production. Since the MCB is the common and only starting source, it can be exhaustively characterized with regard to contamination by adventitious and endogenous agents.