Doppler methods are unique among clinical techniques in ultrasound in that they have the potential to offer information related to the function of an organ beside its morphology. After describing the Doppler effect, the instrumentation with both continuous and pulsed waves is precisely considered, together with the duplex scanner. The multigate pulsed system and the color-Doppler flow imaging are also described with their advantages and disadvantages. The analysis of the Doppler spectrum covers a large part of the paper, analysing the spectral display and the factors affecting content of the spectrum. The author stresses then the interpretation of color-Doppler images, the quantitative methods in Doppler, the addition of color hemodynamics and physics of blood flow and the abundant Doppler artifacts. Bioeffects considerations and future technical developments are then examined; in the final chapter the author describes the new instrumentation, quantitative measurements, Doppler tissue characterization.