The dance along the artery The circulation on the lymph Are figured in the drift of stars. T. S. Eliot Die Methode ist alles. Carl Ludwig In physiology a spirit of finesse is required. Claude Bernard Armed with modern Doppler instrumentation, scientists can now quantify the red blood cell's "dance along the artery" as well as "the drift of stars. " In disciplines of science and medicine ranging from cardiology to astronomy, the Doppler principle now provides invaluable velo city measurements in the microcosm of capillary beds and in the cosmos. The newest appiication of the ubiquitous Doppler principle, laser-Doppler velocimetry, has been used to measure blood ftow in tissue for just a few years, but we perceived that, like most new techniques, the birth of laser-Doppler blood ftowmetry was not easy, nor was it likely to pass through infancy and reach maturity without difficulty. In physiology and medicine, better techniques for measur­ ing blood ftow are constantly in demand, but they often exhibit an unfortun­ ate boom-and-bust cyde: widespread acceptance and uncritical use are soon xiii xiv Preface followed by studies delineating the limits of the method's validity. The technique is then abandoned for the next more fashionable one, thus proving Ludwig's dictum that a given method is everything or nothing depending upon whether one can believe the data it yields.