This chapter discusses fat embolism syndrome: adult respiratory distress syndrome. Fat embolism syndrome (FES) occurs when fat droplets become impacted in pulmonary microcirculation and other microvascular beds, especially in the brain and is characterized by respiratory failure, cerebral dysfunction, and petechiae. It is most commonly associated with fractures of long bones and the pelvis, and is more frequent in closed, rather than open, fractures, however, it can also be caused by nontrauma related causes such as acute pancreatitis, sickling crisis, diabetes mellitus, burns, joint reconstruction, liposuction, cardiopulmonary bypass, decompression sickness and parenteral lipid infusion. Multiple theories have been proposed to explain the pathophysiology of FES. Diagnosis is usually made on the basis of clinical findings but biochemical changes may be of value. Gurd and Wilson proposed major and minor diagnostic criteria for FES, a diagnosis of FES could be made if one major feature, four minor features, and fat macroglobulinemia is present.