Osteonecrosis of the jaw, at one time considered to be infrequent has now become a major public health concern not only in the United States, but throughout the world. The wide-spread use of radiotherapy for head and neck cancer as well as bone antiresorptives and antiangiogenic agents have increased the incidence of osteonecrosis. While the exact pathophysiological process of osteonecrosis is yet to be clearly defined, there has been a much higher incidence of medication-related osteonecrosis of the jaw relative to the other types of osteonecrosis. The traditional osteoradionecrosis still occurs despite better treatment planning and shielding to minimize collateral damage to bone. There are other related necrotic lesions secondary to usage of recreation drugs and the use of steroids. This chapter will give comprehensive information about these different types of bone necrosis; provide the readers with radiographic diagnostic criteria and updates on current theories on pathophysiology of osteonecrosis.