Microbial infection of the dental pulp is the most common cause leading to the Endodontic treatment. Success in Root canal treatment depends on the reduction or elimination of these organisms. Endodontist should have the knowledge about the bacterial species involved in Pulpal and Peri-apical pathosis and the mechanism by which they bring about changes in. Root canal treatment may not result in healing for a number of reasons. The most frequent reasons are failure to eliminate microorganisms that were present at the time of initial treatment or the reintroduction of the microorganisms into the root canal system after initial treatment was completed. the healthy pulp and periodontium. So, knowledge of microbiology is essential to an understanding of basic methods of Endodontic debridement and canal sealing.