This paper is concerned with the use of immobilized enzymes in analysis which has increased tremendously in recent years. The various applications can be categorized by the manner in which the enzyme is used as: transducer bound enzymes; immobilized enzyme reactors; and miscellaneous batch and membrane applications. Enzyme electrodes, making use of either potentiometric or amperometric detection, are the best known example of the first class. Analyses based on reactors have been performed using immobilized enzymes either packed in a column as in liquid chromatography or bound to the inside walls of hollow tubes. In this review, the authors discuss the basic principles of operation of each of these classes with particular emphasis on optimization of the devices. In addition, some applications of immobilized enzymes in environmental, pharmaceutical, and clinical chemistry are presented.