It has proven surprisingly difficult to determine the relationship between neutrophils and pulmonary endothelial cells under normal conditions in the intact lung, let alone under conditions that result in endothelial injury. Some of the issues that bear directly on this problem include the size of the marginated pool of neutrophils in the lung. the distribution of neutrophils in the lung, the conditions governing neutrophil margination in the lung, the effect of margination on neutrophil and endothelial cell function, and the effect of activation on marginated and sequestered neutrophils and pulmonary endothelial cells. All of these issues are addressed in this chapter with particular emphasis on in situ studies of neutrophil-endothelial cell interaction and on issues that, in our opinion. have not been adequately studied as yet.