The defects, which are formed as a result of destruction of alveolar bone due to periodontal disease, are termed osseous defects. Osteoblasts are the primary cells responsible for the synthesis of bone matrix, which later on undergoes calcification. Bone destruction in periodontal disease is caused by osteoclasts and mononuclear cells and is mediated by local and systemic factors. Local factors that are responsible for bone destruction are chronic gingival inflammation, trauma from occlusion, and combination of both. Mechanism of bone formation and bone destruction, diagnosis of osseous defects, factors determining bone morphology in periodontal disease, bone destruction patterns in periodontal disease, and prevalence and distribution of bone defects in moderate adult periodontitis are also discussed in this chapter.