Echo can be used to examine not just the heart’s anatomical structure but also the flow of blood through the heart. This in turn provides valuable information about valvular function, intracardiac shunts and so forth. The study of the heart’s fluid dynamics is made possible by the Doppler principle, discussed in this chapter. As well as allowing the assessment of blood flow, the Doppler principle has also, more recently, been applied to the study of myocardial function (tissue Doppler imaging).