The main symptoms in patients with peripheral arterial occlusive disease (PAOD) are claudication, rest pain, skin ulcers or gangrene. All these symptoms result from a reduction in blood flow in the nutritional capillaries below the demand of that tissue for a certain specific situation. To study the blood flow in the nutritional capillaries of the human muscle is presently not possible. However, the nutritional capillaries can be easily and directly studied by the technique of vital capillary microscopy. Already in the beginning of the twentieth century, August and Marie Krogh opened the road to modern microvascular physiology when they described the diffusion of oxygen through the wall of muscle capillaries, and August Krogh received the Nobel Prize for the achievements in 1920. A few years later (1922), Otfried Müller published his book Die Kapillaren der menschlichen Körperoberfläche in gesunden and kranken Tagen with fantastic colourful drawings of capillaries from almost every part of the skin surface. Even movements of blood cells and plasma gaps in human nailfold capillaries were described.