Successful nonsurgical root canal treatment depends on a thorough knowledge of root morphology and canal systems. Among all teeth, the mandibular premolars may be the most difficult to treat because of the complex canal system and numerous variations in these teeth. One of the most important anatomic variations is the ”C” configuration of the canal system. Once recognized, the complex canal system of C-shape roots provides a challenge with respect to debridement and obturation. This article will review the literature with respect to the etiology, prevalence, roots and root canal morphology, clinical diagnosis and endodontic treatment of the C-shaped mandibular first premolar.