The area of endodontics has expanded in recent years to include operations that formerly were not done. e.g hemisection, intentional or unintentional replantations and treatment of periodontally-endodontically involved teeth. However, treatment of pulpitis and infected root canals occupy the major part of endodontic practice. The radical treatment of pulpless teeth by extraction which was so prevalent a few decades ago has given away to conservative treatment and retention of such teeth. What was brought about this change? Essentially, it has been due to a gradual veering away from the focal infection theory, simplification of strategic pulpless teeth as abutments. Research studies that indicated the pulpless tooth were either bacteriologic or roentgenologic in most of them the method of study was erroneous and unscientific. Roentgenologic studies condemned the pulpless tooth even more than the bacteriologic studies.It remained for an Englishman, E.W.Fish, to clarify the dark points of endodontic treatments by his experiments. he cauterized the gingiva and gingival sulcuses of extracted teeth and proved that contamination of root surfaces was due to above sources.Fish carried out an even more important experiment by establishing foci of infection in the jaws of guinia pigs and found four distinct areas around the focus of infection. These areas are called as zone of infection, zone of contamination, zone of irritation and zone of stimulation, with their special characteristics.From an endodontic stand point this study has considerable significance, as it explains both the disease process occurring in bone and repair following endodntic treatment. The focus of infection is always in the root canal. The periapical tissue is essentially strile except for occasional inroads, despite this focus. As the microorganisms grow out of the root canal and into the periapical tissue they are destroyed by the polymorphonuclear  leucocytes. In such cases where the microorganism overcomes the polymorphonuclear cells, an acute abcess is formed. In most cases however, the reverse is true, the microorganisms entering the bone are destroyed and a chronic abcess developes. Where defence is adequate and periapical bone will be destroyed but a wall of fibrous tissue develops and the lesion is called a granuloma. In some cases the epithelial cell rests in the periodontal membrane are stimulated from a cyst.