Summary: This section is designed to give the reader a concise overview of a complicated subject to serve as a background to the main theme of this issue: how to treat nontraumatic osteonecrosis (ON) of the adult hip. It reviews the etiology and pathogenesis of ON, the clinical picture, and the importance of imaging techniques. Three of the most commonly used systems for classification and staging are described. The role of pharmacologic agents is discussed. Surgical procedures are generally divided into 2 categories: those instituted during the earlier stages, when the goal is to retard or halt progression of ON in order to preserve the femoral head; and those which are indicated after femoral head collapse and acetabular damage has taken place, and which include various arthroplasties and reconstructive procedures.