Recent advances in diagnostic imaging, especially in magnetic resonance imaging, have enabled early diagnosis of osteonecrosis, before radiographic changes and femoral head collapse. This is of clinical importance because it allows prompt institution of prophylactic treatment, thus improving the outcome. Although it is now recognized that the extent of the necrotic lesion is a significant factor in determining prognosis and treatment, most published reports on the treatment of this condition have not attempted to correlate outcome with lesion size. This report reviews the different imaging modalities available for the diagnosis and evaluation of osteonecrosis and summarizes their accuracy and clinical role. It describes different methods for staging avascular necrosis as well as techniques for estimating or measuring lesion size. The close correlation between lesion size and outcome is emphasized. These findings encourage the use of a method of evaluation and staging that includes a specific determination of lesion size as well as stage.