Modern research into the physiology of the blood circulation has been influenced decisively by Carl Ludwig. His contributions to the special field of microcirculation are typical examples of his scientific mode of thinking, being characterised by exactness and richness of ideas. His demand for a useful division of the complete circulatory system into two parts, namely the intra- and extravascular circulation, has remarkable heuristic value. His ideas concerning the functions of the extravascular space have maintained their validity up to the present day. His conception of the existence of open connections between the interstitial compartments and the inside of the lymphatic vessels was far ahead of contemporary knowledge: the relevant findings and ideas of Ludwig look, in part, like "confirmations" or anticipations of modern investigation results. This is illustrated by corresponding examples. As far as his scientific conceptions are concerned Ludwig was very particular to abstain from admission of any hypothetic throughts into them. The high reliability of his conceptions resulting from this custom is exemplified by comparing his interpretation of fluid and substance exchange in the terminal vessel area with "Starling's law" as the currently accepted explanation of these processes.