Perfusion systems for animal cell cultures are increasingly used for high cell density processes to enhance the productivity of bioreactors. There are four different process modes that can be used in animal cell cultivation: batch, fed batch, continuous culture, and perfusion culture. In both batch and fed batch, the metabolite concentrations cannot be kept constant and the accumulation of catabolites like lactate and ammonium limits the process duration to about ten to 15 days. In contrast, with continuous and perfusion processes, there is a constant influx of fresh cultivation medium and a corresponding removal of recombinant protein and catabolites. The concentrations of metabolites remain relatively constant and the process duration is not limited by the buildup of waste products. This extends the duration of these cultures to several weeks or months. The resulting steady-state conditions for metabolites can enhance cell-specific productivity and product quality, for example, by improved glycosylation or reduced aggregate formation. In continuous culture, cells are removed in the effluent and this limits the product output per liter of bioreactor volume (volumetric productivity). In contrast, substantially higher cell concentrations are attained in a perfusion system because the cells are retained in the bioreactor resulting in increased volumetric productivity. A five- to 20-fold improvement over batch cultivation has been reported for perfusion cultures…