In the metabolism of almost all human cells, a sequential addition of electrons to oxygen leads to the formation of reactive oxygen species (ROS). ROS have been implicated in more than 100 diseases and may be the common denominator in the pathogenesis of the most important health problems facing the world today. The last decade has been characterized by a progressive increase in the understanding of oxidant chemistry and the role of ROS in pulmonary disease. The majority of deaths among critically ill patients are the result of sepsis and its sequelae, including acute respiratory distress syndrome (ARDS). Nurses must understand the processes involving ROS that are in play when they are caring for patients with ARDS. This article describes what is known about the formation of ROS, the pathophysiology of ARDS, and the role ROS play in the pathogenesis of ARDS.