Design and selection of cell culture bioreactors are affected by cell-specific demands, engineering aspects, as well as economic and regulatory considerations. Mainly, special demands such as gentle agitation and aeration without cell damage, a well controlled environment, low levels of toxic metabolites, high cell and product concentrations, optimized medium utilization, surface for adherent cells, and scalability have to be considered. This chapter comprises engineering aspects of bioreactor systems (design, operation, scale-up) developed or adapted for cultivation of mammalian cells, such as bioreactors for suspension culture (stirred-tank reactors, bubble columns, and air-lift reactors), fixed bed and fluidized bed reactors, hollow fiber and membrane reactors, and, finally, disposable bioreactors. Aspects relevant for selection of bioreactors are discussed. Finally, an example is given of how to grow mammalian suspension cells from cryopreserved vials to laboratory and pilot scale.