Acute lung injury is defined as a syndrome of inflammation resulting in increased capillary-alveolar permeability that is associated with a constellation of clinical, radiological and physiological abnormalities. Adult respiratory distress syndrome (ARDS), relatively common in the intensive care, is a more specific, severe form of ALI, with diverse causes. Diffuse alveolar damage is the pathognomonic feature of the syndrome. Under the pathophysiologic conditions of increasing capillary membrane permeability, large macromolecules can gain access to the interstitium, resulting in pulmonary interstitial edema, and from there enter the alveolar space in severe cases. Detecting the distribution of macromolecules in lung tissue, therefore, constitutes evidence of the injury. Traditional pathologic examination of biopsy/autopsy materials is the only route to provide pathologic evidence of the capillary-alveolar membrane damage induced lung injury. Although morphometric analyses are more laborious, they provide qualitative assessment of morphological changes that is an important aspect of acute lung injury, see Fig.l.