The introduction of Doppler guide wires has allowed the wide-spread use of Doppler technology in the catheterization laboratory for coronary diagnostics and pathophysiological studies. Doppler-ultrasound-derived measurement of coronary flow velocity serves as a substitute for true volumetric flow measurement. To produce reliable and reproducible flow velocity data, the whole Doppler spectrum should be evaluated. Special attention should be paid to the velocity distribution within the spectrum. A spectral display with strong signals in the high velocity range and a sharply defined envelope are markers for a good positioning of the Doppler wire. Additional security for the optimal positioning can give the recently developed tracking indicator. For reliable CFR determination using average peak velocity at rest and during hyperaemia, changes of the shape of the velocity profile and of the cross-sectional vessel area, as well as the position of the Doppler guide wire, have to be taken into account, otherwise the CFR will be underestimated. To eliminate cross-sectional area changes, the vessel should be pretreated with nitroglycerin.