Osteonecrosis (avascular necrosis, bone infarction, aseptic necrosis) is a phenomenon where bone and bone marrow cells go to necrosis as a result of ischemia. Osteonecrosis involves a process in which dead cell resorption and new bone formation are seen together. Whatever the main cause in pathophysiology, the common result is that subchondral microcirculation is affected. Trauma (as a result of fractures or dislocations) and direct damage to vascular structures, vascular occlusion due to thrombus or fat embolism, or extravascular compression as a result of intra-osseous lipocyte hypertrophy are usually blamed for ischemia. Knowing the pathophysiology and possible risk factors in osteonecrosis will be important for early diagnosis and joint sparing approaches.