Obtaining a correct working length is critical to the success of endodontic therapy. Failure to identify this crucial measurement can result in untoward treatment outcomes, which may include increased patient discomfort, possible infection or cyst development, and extrusion or intra-canal medication into the periradicular tissue. This article reviews the classical and current terminology, philosophies, and techniques that are used at present to determine endodontic working length accurately, and describes their clinical application. Recent clinical advances such as the electronic apex locator, direct digital radiography, and the surgical operating microscope also are discussed.