The medical ultrasound, the Doppler frequency shift is the difference between the frequency of the transmitted and reflected ultrasound. This is due a relative movement between the reflector (most frequently the red blood cells) and the ultrasound transducer. The ultrasound apparatus registers this difference in frequency and calculates the linear rate of flow employing the Doppler equation. The Doppler analysis is presented acoustically, graphically or by means of a colour code. In pulsed Doppler technique, conventional ultrasound scanning can be combined with Doppler analysis. Important indications are: differentiation between vascular and non-vascular structure, documentation of flow and determination of the direction of flow, diagnosis and quantitation of arterial stenoses and assessment of transplants.