Industrial processes involving animal cells for the production of useful products still seem to be rather uncommon. Nevertheless, during the last four decades of the last century the number of relevant processes has increased from production of virus vaccines to monoclonal antibodies and finally complex structured glycoproteins. As soon as cell lines became permanent and culture medium changed from purely biological fluids to more or less defined chemical media, large-scale cultivation could begin. The developments of the 1970s - fusion of cells to form hybridomas, and genetic engineering -triggered a second wave of products. Monoclonal antibodies and recombinant proteins for diagnosis and therapy set new challenges for the inventors. Historically, there has been no straightforward process development since the product dictates the process operation. Therefore, the scale of production covers the whole range from small multiple-unit reactors (flasks or roller bottles) up to 10,000-1 single-unit batch reactors. Products with high value and small demand can be produced in multiple-unit systems whereas "bulk" products for vaccination and therapy may need large-scale bioreactors to be cost effective. All the different systems have their advantages and disadvantages and significant challenges that curb the development of effective perfusion cultures still remain.