In organic chemistry, students struggle to write and understand mechanisms for reactions. Since, we are actively engaged in synthesis and biological applications of heterocyclic compounds, we intend to write a textbook on chemistry of heterocyclic compounds giving emphasis on mechanisms of almost all types of reactions. Hence, this work will motivate the students to read and understand various organic reactions. It has been suggested to refer the appropriate sections to understand the reaction mechanisms. We have extracted contents from research articles, reviews and refered some book chapters, which have been cited at appropriate places throughout the textbook.