The accuracy of working length determination in root canal treatment plays a crucial role in the treatment and prevention of periapical disease. Since the introduction of the first apex locator by Sunada in 1961, new and more accurate devices have been developed. Electronic apex locators reduce the number of radiographs required and may detect root canal perforations and fractures. Third and fourth generation apex locators can operate in the presence of intact or necrotic tissue, blood, inflammatory exudates, and various irrigants. The Raypex® 4 and the Apex NRG® novel apex locators are discussed in detail. Schlagwörter: apex locators, endodontics, root apex, working length, Raypex® 4, Apex NRG®