Color Doppler M-mode echocardiography provides a spatiotemporal map of blood distribution (v(s,t)) within the heart, displaying (typically) a temporal resolution of 5 msec, a spatial resolution of 300 μm, and a velocity resolution of 3 cm/sec. M-mode echocardiographic data can be obtained along a streamline from the mid-left atrium to the mid-left ventricle from either the apical transthoracic window or the basal transesophageal window. A key parameter of the color Doppler M-mode is the transmitral propagation velocity, the slope of the leading edge of the M-mode derived E-wave. This propagation velocity is significantly less than the velocities measured within the E-wave by pulsed Doppler echocardiography and appears to provide information about diastolic function which is quite distinct. Published studies have indicated that color M-mode propagation velocity is inversely related to the relaxation time constant and is reduced in settings of acute myocardial ischemia. This manuscript will outline physics and instrumentation issues involved in the use of the color Doppler M-mode, discuss prior applications of this technique, and postulate on its role in the assessment of ventricular diastolic function.