Locating the appropriate apical position always has been a challenge in clinical endodontics. The electronic apex locator (EAL) is used for working length determination as an important adjunct to radiography. The EAL helps to reduce the treatment time and the radiation dose, which may be higher with conventional radiographic measurements. According to recent publications, the accuracy of frequency-dependent EALs appears to be much higher compared with traditional-type EALs (simple resistance type or impedance type). This article reviews the history and the working mechanism of the currently available EALs, and suggests the correct usage of the apex locator for a better canal length measurement.