The main function of the bones is the internal support and the storage of inorganic ions. Bones consist of the outer compact bone that harbors the inner cancellous bone and the bone marrow. Main cells are the bone-building osteoblasts, the bone-supporting osteocytes, and the bone-resorbing osteoclasts. All the cell lines communicate via the OPG RANK RANKL system.The term osteomyelitis describes inflammation of all bone structures. Depending on the clinical presentation and the cause triggering the osteomyelitis, it can be classified into acute osteomyelitis that is called secondary chronic osteomyelitis after a time span of more than 4 weeks. In general, an odontogenic infection is responsible whereas the trigger of the secondary chronic osteomyelitis is unknown. Symptoms of acute osteomyelitis are fever, listlessness, swelling, pain, reddishness, trismus, tooth mobility, pus, and paresthesia. In the secondary chronic stadium, the swelling is less prominent and a periosteal reaction, sequester formation, and fistulas can occur. The primary chronic osteomyelitis often has few and not very prominent symptoms with a nonsuppurative inflammation. Therapy consists of antibacterial mouth rinses and antibiotics to different surgical procedures.Osteoradionecrosis and medication-associated osteonecrosis are subentities where the radiation or a medication alters the bone biology and subsequently causes osteonecrosis of the affected bones.