A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different elements as members of its ring(s). Heterocyclic chemistry is the branch of organic chemistry dealing with the synthesis, properties, and applications of these heterocycles. This text is a concise book that gives details of heterocyclic compounds. This book will also be useful to the students preparing for various competitive examinations. Much emphasis has been placed on chemical reactions and mechanisms of heterocyclic compounds. Each compound had been described in a clear and systematic manner. The subject-matter presented in each book, though concise, has adequate coverage of this subject; the important points wherever necessary have been highlighted; complex portion of the content has been interpreted in an easy to grasp manner; and long sequences of references of reactions have been summarized in short run flowcharts.