Apical periodontitis is an inflammatory lesion in the periodontal tissue that is mostly caused by bacteria from the infected root canal. Apical periodontitis will become a chronic disease without proper endodontic treatment. Single lesions of apical periodontitis may or may not be present with clinical symptoms including pain, tenderness and swelling. Even though periapical bone resorption represents tissue destruction, it occurs as a part of the organism defense mechanism. Different structural frameworks of apical periodontitis can be identified on a microscopic level. These forms include apical granuloma, apical abscess and apical cyst. Microbial infection is the main precondition for the occurence of apical periodontitis in a non- treated tooth. All necrotic teeth associated with a periapical radiolucency and a sinus tract had bacterial infection in the apical root canal system in the form of a biofilm. A proper examination, pulp vitality test, radiographic findings and an attentive questioning of the patient about the symptoms may help identify a dental etiology. Treatment must be focused on the elimination of the source of the infection. Appropriate cleaning, shaping, and sterilization of the root canal and of the periradicular region, as well as root canal filling, determine the success of the treatment and periapical healing. Treatments such as surgical excision, biopsy, revision surgery and radiotherapy, along with the administration of antibiotics, are inappropriate. Communication between the dentist and the physician is suggested to prevent misdiagnosis and treatment resulting in a persistent infection.