Mandible is one of the most sophisticated bones in human body. Defects of mandible can be caused by tumor, cyst, trauma, infection, and osteoradionecrosis (ORN) . Different defect situations may result in different functional deficits and reconstructive complexities. A logical classification system of mandibular defects is of great importance for reconstruction guidance, preoperative evaluations, and prognostic predictions. 17 classification systems have been proposed since 1974, however, none of them is universally used. This article will give us a brief comparison and review on existing classifications, and make a further discussion on the features of an ideal classification system.