Apical periodontitis is an inflammatory disease affecting the supporting tissues of the teeth. It is caused by an endodontic infection, i.e., the spread of necrotic content from the root canal space into the periradicular tissue. The severity of infection is determined by the balance between the hosts defense mechanisms on the one hand and the microorganism patogenicity on the other. Depending on which of these factors predominates, the response may be accute or chronic, and the clinical presentation may vary. Bacteria reside deep in the root canal system, in hard-to-reach places, where host defense mechanisms and the efficacy of systemic antimicrobial therapy are lacking. Due to these characteristics of endodontic infections, the only successful therapeutic method is chemo-mechanical treatment and obturation of the canals and the creation of a final restoration to prevent re-infection and ensure aesthetics, functionality and survival of teeth. Several factors can potentially jeopardize the outcome of endodontic treatment during therapy. When evaluating the outcome, it is important to consider the clinical and radiological criteria. There are numerous reasons for treatment faliure. Some of them, such as the anatomical structure of the canal system, the immune response of the patient's body, and the presence of resistant bacterial biofilms, cannot be influenced. Procedural and diagnostic errors, on the other hand, are solely the result of the therapist's wrong assessment. Proper diagnosis, correct treatment protocol implementation, and complication prevention are critical for the successful outcome of endodontic treatment.