The goal of endodontic treatment is to prevent the development of periapical periodontitis or to achieve appropriate conditions for successful healing of already existing lesions. In order to achieve previously stated goals, it is important to properly carry out all treatment steps: chemo-mechanical canal treatment, three-dimensional canal obturation and coronary restoration. During the execution of these steps, a number of factors can potentially compromise and negate the success of endodontic therapy. Factors that can affect the outcome of endodontic therapy are classified into three groups: biological factors, factors related to the endodontic treatment procedure, and postoperative factors. It is proven that the presence of a preoperative periapical lesion, the quality of the filling and the quality of the coronary restoration significantly affect the success of the treatment. The influence of other factors is less significant, but not negligible. A successful endodontic treatment is considered to be one in which there are no clinical signs of inflammation and a normal periodontal crack is observed radiologically. Paying attention to all of factors, precise diagnosis and correct execution of the treatment can lead to a shift in the balance in the favor of the host's immune response and suppression of infection.