Enzyme catalysed reactions are involved in many of the changes, beneficial or otherwise, that occur in food between the farm gate and the plate. Food chemists need to understand the essential characteristics of enzyme catalysed reactions. These include the dependence of enzyme specificity, inhibition and pH effects on the structure of the active site and effects on proteins generally. Most of the prosthetic groups and coenzymes involved in enzyme reactions are derived from B-vitamins and amongst the metal ions similarly involved are many nutritionally classified as essential trace metals. Many pesticides are recognised as specific enzyme inhibitors. Enzymes naturally present in raw produce are often essential to the formation of the edible end products or unwanted off-flavours etc. Examples include amylases, phenolase, lipoxygenase, alliinase and calpains. Purified enzymes, usually obtained from microbial fermentations, are employed in a wide range of food processing operations. They are of particular value in the synthesis of optically active ingredients such as l-cysteine and aspartame. The chapter concludes with a list of specialist books and review articles for further study.