The alveoli are fibroelastic partitions in the airways of secondary lobules. One side of the basal lamina is invested by capillary endothelium, the other by a continuous epithelium of type I alveolar cells which is probably covered by liquid surfactant system. Alveoli are protected by generations of self-clearing mucous covered airways. Alveolar injury produces swelling of endothelial cells, trapping of platelets and leukocytes and their invasion of interstitial areas and alveolar spaces. Subsequently type I cells and the fibroelastic framework are destroyed. Proliferation of fibroblasts and great alveolar (type II) cells are major repair responses. Conceptual and mechanical analogues and living replicas from enzymes, single cells, and simple lungs to mammalian lungs are useful models. Perturbations of models by one or more agents are used to test agents for toxicity, to mimic effects in man, and to study the mechanisms of disease. Further investigations of alveolar injury and repair are needed.