At the beginning of the XX. century, Gottlieb stressed the importance of cementopathia in the pathogenesis of periodontium. Today we consider that cementum plays an important role in guided tissue regeneration. The aim of this study was to present the histological features of the healthy and diseased cementum. We studied untreatable extracted teeth, prepared for histology in different ways. In the cervical part, the cementum was thin, in the apical and interradicular regions it was thick. We had seen both acellular and cellular cementum. The histopatologic changes were: necrosis, hypercementosis, caries, and in the pulp near the necrotic cementum: odontoblastic lesions. The acquired data about the histology of healthy and diseased cementum lead to the understanding of physiological and pathological events of periodontium. This knowledge also helps us in treatment planning, and assures a base for the improvement of guided tissue regeneration.