Although the pulmonary capillaries were discovered in 1661, the ultrastructure of the wall was not elucidated until 60 years ago. Electron micrographs then showed that only 0.2 μm of tissue separated the capillary endothelium from the alveolar space over much of the area. In retrospect this vanishingly small protective layer should have alerted physiologists to the potential fragility of the capillaries, but this was not appreciated until almost 40 years later. This predicament is unique to pulmonary capillaries. No other capillaries in the body are shielded from the outside environment by such a minute amount of tissue. Reasons why the fragility of the capillaries was not recognized earlier include an inappropriate comparison with the properties of systemic capillaries, the mistaken view that the pulmonary capillary pressure is always low, and a misleading use of the Laplace equation. Evidence for the fragility comes from physiological, pathological, and laboratory observations. As expected from evolutionary considerations, the fragility only becomes evident in the normal lung under exceptional conditions. These include elite human athletes at maximal exercise and animals that have developed the capacity for extreme aerobic activity. However, lung and heart diseases frequently cause capillary disruption. Remodeling of pulmonary capillaries occurs in humans in whom the capillary pressure rises over a long period. Neonatal capillaries are extremely fragile, presumably because they have never been exposed to increased transmural pressures. The capillaries conform to the general biological rule that tissue adapts its structure to carry out its required function.