The causes of osteonecrosis are varied. The pathogenesis of osteonecrosis subsequent to trauma is well known. Furthermore, an association of this disorder with cortisone, haemoglobinopathies, rare metabolic diseases, and risk factors seems to be evident. However, a sizable proportion of patients have osteonecrosis in which none of these associations exists, referred to as idiopathic osteonecrosis. Generally, intraosseous vascularization represents the common pathway for the various causes of the disease. The prognosis is influenced by the age of the patient, the location of the necrosis and the stage of the disease at the time of diagnosis. In recent times diagnosis has been improved by the use of modern imaging modalities, and MR imaging is now widely accepted as the imaging method of choice. It not only allows early diagnosis but also yields exact staging information in advanced disease, which is a requirement for adequate therapy.