The aim of this paper is to discuss the relation between root canal treatment and apical periodontitis (AP). AP is caused by micro organisms and is mainly a chronic inflammation. There is some evidence of an effect on the general health in immunocompromised patients. A higher incidence of AP on root filled teeth has been found. A number of preoperative and operative factors influence prevalence of AP on root filled teeth. Rubberdam placement and cleaning and shaping of the root canal during root canal treatment is important to create a clean canal to promote healing or prevent the development of AP. Placement of a good quality root filling and a well sealing coronal restoration are also important for the prevention of reinfection of the root canal. The quality of root fillings differs between clinical and epidemiological studies what is reflected in a lower prevalence of AP in the former.