More than 80% of the commercial value of enzymes is linked to their applications as process catalysts. Hydrolytic reactions conducted mainly with the enzyme dissolved in the aqueous medium has been the traditional way of using enzymes, this technology still representing a major share of enzyme processes. However, in recent decades the use of enzymes in organic synthesis has widened its scope of application to unprecedented levels. Enzyme reactors can operate batch-wise or continuously; fed-batch operation has also been proposed (Kumar et al. 1996). Batch processes with the enzymes (usually hydrolases) dissolved in an aqueous reaction medium, despite its wide application have several drawbacks, since enzymes are poorly stable and hard to recover in such systems, leading to low productivity; besides, such processes are characterized by a rather low added value so that process optimization is critical for being and keeping competitive. Poor stability is usually the limiting factor in any enzyme process so that enzyme stabilization during reactor operation is a major concern (Ballesteros et al. 1998; O'Fágáin 2003) and among the many strategies for enzyme stabilization (Illanes 1999) enzyme immobilization is the most relevant (Guisán 2006). Immobilized enzymes can be used in batch processes but in this case the enzyme is recovered to be used in subsequent batches until the accumulated inactivation makes necessary to replace the spent biocatalyst. As a consequence, specific productivity (mass of product/mass of biocatalyst time of operation) is increased and bioreactor design becomes flexible to suit the particular needs of a given process.