In this module, we discuss the application of the Doppler effect in color Doppler imaging, including color Doppler gain, color wall filters, color persistence, and color priority. As you will see, color Doppler has some characteristics similar to spectral Doppler, some characteristics similar to 2D imaging, and some characteristics that are unique just to color Doppler imaging. Finally, we will concentrate on the ability to distinguish flow direction from a color image, regardless of transducer format, vessel tortuousity, and aliasing.