Oxidative stress is implicated in the pathology of numerous diseases of the lung. These include cystic fibrosis, chronic obstructive airway disease and asthma. All these conditions are characterised by an imbalance between the amounts of reactive oxygen species (ROS) and available antioxidant defences. In the lung, ROS arise from endogenous sources, such as the influx of inflammatory cells or exogenous sources, such as from air pollution and cigarette smoke. When ROS production increases the redox balance of the airways alters, and this can lead to bronchial hyperactivity and further inflammation. The lung, like many other tissues, has a range of antioxidant defences which help to maintain a balanced redox status. These antioxidants are present in the intracellular, the vascular and extracellular respiratory tract lining fluid (RTLF) compartments. The reduced glutathione (GSH) content of RTLF is particularly high and new findings are beginning to reveal the role that the RTLF GSH pool plays in defending the lung.