This article reviews some of the important physical principles involved in Doppler ultrasound (US) in the context of modern diagnostic US technology. Methods of quantitative Doppler US data analysis used to distinguish normal hemodynamics from disturbed flow conditions are discussed with the aid of numerous examples. Definitions and measurement techniques are presented for determining from Doppler signals such indicators of arterial disease as velocity, volume flow rate, flow impedance, and pulsatility. Spectral analysis and its utility in Doppler signal analysis are described. The various options available on modern Doppler US instrumentation are reviewed, including duplex Doppler techniques and color Doppler flow mapping. Finally, the authors discuss artifacts and pitfalls likely to be encountered in the routine clinical use of Doppler US for diagnostic purposes.