There has been continuous growth in mammalian cell culture bioprocessing. This is primarily driven by the expansion of therapeutic protein production in the pharmaceutical industry. Cell culture medium consists of basal medium and growth supplements. Traditional cell culture medium contains up to 15% animal serum, in addition to its basal medium. A chemically defined cell culture medium typically includes glucose, amino acids, salts, nucleotides, and vitamins. Modern industrial cell culture processes have largely eliminated serum and employ medium free of animal components, to minimize the risk of animal viruses or prior contamination. Virtually all continuous cell culture processes used in biomanufacturing employ cell recycling. With increased equipment reliability and the demonstration of success, there has been increasing interest to explore the use of continuous culture in biomanufacturing.