Heterocycles represent a large class of organic compounds and play an important role in all aspects of pure and applied chemistry. Heterocyclic compounds are widely distributed in nature and essential to life; they play a vital role in the metabolism of all living cells. A large number of heterocyclic compounds, both synthetic and natural, are pharmacologically active and are in clinical use. Heterocyclic chemistry deals with heterocyclic compounds which constitute about sixty-five percent of organic chemistry literature. This book is intended for advanced students, graduates, and researchers from both academic and industry working in the area of heterocycles and organic chemistry and looking for a survey on the new synthetic methods, chemistry, and applications of heterocycles. The classification has been made according to the size of heterocyclic ring. Due to a substantial number of publications on the synthesis and chemistry of five and six-membered heterocycles, it was decided to focuses on the synthesis of them in Chapters 4 and 5, respectively. Chapter 6 summarizes data on pharmaceuticals containing heterocycles.