Enzymes are bio-catalysts which effect transformation of substrates to products with high specificity. The usage of enzymes in domestic and industrial applications is well known and has been well documented since the early history of civilization. With the advances in understanding of enzymology, usage of enzymes in industrial and biotechnological pro­ cesses and molecular medicine has proliferated. One of the key factors in the widespread application of enzymes in modern technologies is the development of enzyme immobilization tech­ niques, which overcome certain practical, functional and economic con­ straints. Many natural enzymes can be stabilized by immobilization on solid matrices, with most of the activity retained, for a variety of applica­ tions. An important application of immobilized enzymes is in liquid chromatography. In the last decade, post-column enzyme detection has become established as an important discipline in liquid chromatography. The new detection approach offers more sensitive and specific ways for measuring major classes of biomolecules. Reactors are fabricated by packing the immobilized enzymes into small columns, which can be placed immediately after an HPLC column.