This book edition gives a general introduction to the Role of Reaction intermediates in different Organic Reactions. The most common reaction intermediates which we usually come across are Carbocations, Carbanions ,Free radicals, Carbenes, Nitrenes or Benzynes. The Ylides are another class of reactive intermediates.Different types of ylides like Nitrogen,Phosphorus and Sulphur have been explained and a brief description on the applications of various reaction intermediates along with the mechanism of specific named reactions and rearrangements has been discussed in this edition of book .It is designed primarily for the undergraduate and postgraduate students of Organic Chemistry.