Osteonecrosis may afflict over 20 million patients worldwide. Prevention and treatment of osteonecrosis is dependent on a better understanding of the pathogenesis of the disease. Treatments range from observation with behavior modification to total joint replacement. As osteonecrosis patients are often relatively young, treatment options sparing the joint and reducing or delaying the need for joint replacement are essential. The results of joint sparing procedures are generally better if performed at early, precollapse stages. Approaches to treatment of early-stage disease are based upon the clinician's acceptance of one of the many hypotheses regarding the underlying pathophysiologic mechanisms involved. These mechanisms have been categorized as direct effects on cells or tissues, vascular interruption, intravascular occlusion, and intraosseous extravascular compression. While there has been a substantial increase in research regarding osteonecrosis, many questions remain to be answered concerning risk factors, pathophysiology, and nonsurgical and surgical interventions.