The femoral head is the main location of avascular osteonecrosis. The lesion remains asymptomatic for several months or years before causing non specific hip pain. Risk factors have been identified, mainly femoral neck fractures, corticosteroid therapy and related conditions (lupus erythematosus, organ transplantations), alcohol abuse, dyslipidemia, sickle cell disease, HIV infection, caisson workers, Gaucher's disease, male sex. When typical radiological signs are lacking, MRI is the best investigation. Progression toward hip joint damage highly depends on the necrotic volume assessed at MRI. The combination of plain radiographs which help staging the severity of osteonecrosis, and MRI which indicates the prognosis of the lesion, determines the therapeutic options: symptomatic pain relief therapies or surgical treatment (core decompression, osteotomy or total hip replacement).