Osteonecrosis (avascular necrosis, aseptic necrosis or subchondral avascular necrosis) is not a specific disease entity but the final common pathway of a number of conditions leading to bone death. The main predisposing factors (trauma, glucocorticosteroids, alcoholism and connective tissue disorders) should be carefully sought, but osteonecrosis can also be idiopathic in origin. Its most common localization is to the femoral head, followed by the humeral head, the knee and the small bones of the wrist and foot. New imaging techniques, especially magnetic resonance imaging, have improved the early diagnosis of osteonecrosis as radiographs may be normal in the initial stages. The outcome for the patient is influenced by many factors, the size and localization of the bone necrosis being of primary importance. Early intervention (both surgical and non-surgical) has definitively improved the outcome, but still nearly 50% of cases of femoral head osteonecrosis necessitate arthroplasty. In this chapter, we shall concentrate on patients at high risk of osteonecrosis, on evidence-based diagnosis, especially in the early stages, and on evidence-based treatment suited to the staging system of osteonecrosis.