In most clinical studies, evaluation of left atrial (LA) function evolved from estimation of LA size on chest radiograph, electrocardiographic P wave abnormalities, LA diameter determined at fluoroscopy or by echocardiography, LA pressure recordings, blood flow velocity with Doppler echocardiography, and measurements of LA volume based on echocardiographic, cineangiographic, radionuclide, and magnetic resonance imaging techniques. The recent development of real-time two-dimensional echocardiographic acoustic quantification (AQ) suggests that LA dimensions can be measured instantaneously to provide online assessment of its systolic and diastolic functions. By means of AQ echocardiography and simultaneous recordings of LA pressure, the LA pressure-area relation can be obtained. LA pressure-area relation consisted of two loops: the A loop, representing the LA pump function, and the V loop, representing LA reservoir and passive emptying functions. The importance of LA function has been demonstrated in congestive heart failure, atrial fibrillation, hypertension, and ischemic heart disease and during pacing or dobutamine infusion.