Depending on the causes, the components of the alveolar structure (the epithelium and the alveolar space, the interstitium, and the pulmonary capillary endothelium) can be involved differently and can serve as primary targets of the underlying pathological processes. Based on history, clinical presentation, BAL data, and, most important, on information from lung tissue studies, the disorders can be gathered in groups according to predominant structural targets (Figure 4 ).