Enzymes show extraordinary specificity in catalysing biological reactions. A systematic classification of enzymes has been adopted on the recommendation of the International Enzyme Commission. The new system divides enzymes into six major classes, which are subdivided further into subclasses according to the type of reaction catalysed. For example a recommended name, a systematic name and a classification name for phosphodiesterase is phosphoric diester hydrolase while its classification number is EC 3.1.4. Research into soil enzymes has increased steadily over the last 30 years. Various activities associated with biotic and abiotic components contribute to the overall activity of soil enzymes. According to some scientists an enzyme may be associated physically with proliferating animal, microbial, and plant cells and it may be located in the cytoplasm in the periplasm of Gram-negative bacteria or attached to the outer surface of cells. They can also be present in non-proliferating cells (for example, microbial spores or protozoan cysts), in entirely dead cells or in cell debris. Other enzymes are present as an extracellular soluble molecule. They can also be temporarily associated in enzyme-substrate complexes, adsorbed to clay minerals or associated with humic colloids. Some of these categories according to experts may represent various stages in the life of an enzyme. An intracellular enzyme may still function after the cell dies and thus it becomes associated with cell debris. It may be released in the aqueous phase and may eventually be adsorbed in an active form by soil colloids. Enzyme-clay and enzyme-organic polymer complexes show remarkable resistance to proteolytic and thermal denaturation. Several methods exist for the measurement of enzyme activities. Experts cautioned about the interpretation of results arising from measurement of soil enzyme activities. According to these authors these measurements represent the maximum potential because the incubation conditions for enzymes assays are chosen to ensure optimum rates of catalysis. Thus the concentration of substrate is in excess and the optimum values of pH and temperature are selected to permit the highest rate of enzyme activity. Also the volume of the reaction mix is such that it allows free diffusion of substrate. Hence the problems arising from the interpretation of measured soil enzyme activity have often led to the conclusion that soil enzyme assays have no meaning in ecological and agricultural terms. Enzymes measurements do answer qualitative questions about specific metabolic processes, and in combination with other measurements (ATP, AEC, CO2 evolution), may increase the understanding of the effect of agrochemicals, cultivation practices, and environmental and climatic factors on the microbiological activity of soil. For example, some scientists discovered that enzyme activity was higher in the uppermost 20 cm of soil in plots tilled by shallow ploughing; this was not the situation in soils tilled by deep ploughing. This book explores in detail enzyme measurements as it affects understanding of land management practices.