Heterocyclic compounds are found in abundance in nature and hold an immense significance to life as they form a part of constitution of variety of natural compounds viz. vitamins, hormones, antibiotics. Hence, they have attracted attention in the design of biologically active molecules. Further, heterocycles constitute a highly important class of Organic compounds which has numerous applications in the medicinal field. Heterocyclic compounds have been an integral part of the development of new biologically active molecules, i.e. drug design or drug discovery and drug development. The book tries to summarize role of heterocyclic compounds in the drug discovery process as well gives an account of heterocycles being used in clinical practice for various diseases.