Because of their excellent functional properties (activity, selectivity, specificity), enzymes have a great potential as industrial catalysts in a number of areas of chemical industry: fine chemistry, food chemistry, analysis and so on (Koeller and Wong 2001). However, the enzymes have been modified during evolution to optimize their behavior in the framework of complex catalytic chains inside the living cells under stress and subjected to regulation. Obviously, enzymes have not been optimized by evolution in order to work as catalysts in industrial reactors so that some of their properties are not well suited for that purpose: they are water soluble, unstable at conditions different from physiological, frequently inhibited by substrates and products of reaction and have rather narrow substrate specificity. In most cases, enzymes have to be greatly improved for their application as industrial catalysts. The engineering of enzymes for such purpose is one of the most exciting, complex and interdisciplinary goals of biotechnology, considering different techniques like: a) the screening, inside the biodiversity, of enzymes with improved properties; b) the improvement of enzyme properties via techniques of molecular biology; c) the improvement of enzyme properties via immobilization and post-immobilization techniques; d) the improvement of enzyme properties via reaction and reactor engineering. These techniques complement each other to succeed in improving enzyme properties for delivering catalysts for a much more sustainable chemical industry, where very complex and useful compounds are synthesized under very mild and cost-effective conditions.