The determination of an accurate working length is one of the most critical steps of endodontic therapy. The cleaning, shaping, and obturation of the root canal system cannot be accomplished accurately unless the working length is determined precisely. The calculation determines how far into the canal the instruments are to be placed and worked and thus how deeply into the tooth the tissues, debris, metabolites, end products, and other unwanted items are removed from the canal. It will limit the depth to which the canal filling may be placed. If calculated within correct limits, it plays an important role in determining the success of the treatment and conversely, if calculated incorrectly may result in treatment failure.