In this lecture we have tried to give a broad account of enzymology and have aimed to put current knowledge into prospective. To study enzymes will be helpful to understand a crucial role of catalysis in metabolic process of living organism. Because of the complexicity of such processes, it is necessary to gain an insight into the properties of enzymes in simpler systems such as isolated entities in the test tube. Next we followed a progression from the properties of isolated enzymes to the behavior of enzymes in complex systems, leading up to cells. We include discussion of the importance of enzymes in medicine and industry to emphasize that enzymology is not purely academic but has increasingly wide applications.