This work sets out to present an up-to-date account of the field of enzymology, describing the properties of enzymes in systems of increasing complexity and showing how these properties can be exploited for clinical and technological purposes. The first six chapters deal with isolated enzymes, discussing their purification, structure, kinetics, mechanism of action and the control of their activity. The behaviour of enzymes in more complex systems, including the intact cell, is then considered. The final chapters cover the turnover of enzymes "in vivo", clinical aspects of enzymes and enzyme technology. Throughout the text, emphasis is placed on the collection and interpretation of experimental data.