Doppler ultrasound exploits the Doppler effect — the change in frequency of a sound wave when the source or reflector is moving relative to the transducer. In clinical practice it is used to assess blood flow direction, velocity, and character. The three main forms are spectral Doppler (pulsed wave and continuous wave), color Doppler, and power Doppler. Doppler is essential in vascular studies, echocardiography, and obstetric imaging.