Recent clinical studies reported that in critically ill patients, oxidative stress caused by an increased production of reactive oxygen species (ROS) and reactive nitrogen species (RNS) and reduced antioxidant response is an expectable event. This book presents the actual scientific available evidence of the pathophysiology of oxidative stress and the role of it in different clinical situations in the critically ill patient, supporting an adjunct therapy to prevent the evolution to a multiple organ dysfunction syndrome. This point of view is supported by many recent clinical and experimental studies.