Immobilization of whole cells has become an efficient tool for biosynthesis, biotransformation and analysis. High cell density, high operational stability, easy handling, propably in continuous systems, and multiple reuse are important advantages of immobilized cells. In comparison to other methods like adsorption, crosslinking and encapsulation, the entrapment within a polymeric network is the most widely applied technique in heterogeneous biocatalysis. For immobilizing mammalian cells nearly exclusively the adsorption onto micorcarriers is used. Requirements for the polymers to be used in such immobilized cell systems are discussed in detail.