Diastole can be divided into four phases: isovolumic relaxation, early filling, diastasis, and atrial systole. The amount of LV filling that occurs during each of these phases depends on myocardial relaxation, the passive characteristics of the LV, the characteristics of the left atrium, pulmonary veins, and mitral valve, and the heart rate. When diastolic function is normal, the net effect of these factors results in an LV filling sufficient to produce an adequate cardiac output, while mean pulmonary venous pressure is maintained below 12 mm Hg. In the absence of systolic dysfunction, abnormal diastolic performance is usually due to abnormal relaxation and/or changes in the passive LV characteristics. Invasive studies can quantitate the rate of myocardial relaxation and the LV diastolic pressure-volume relation. More recently, RNA and Doppler echocardiography have been used to noninvasively evaluate diastolic performance by determining the pattern of LV diastolic filling. At rest, most LV filling occurs early in diastole. Conditions that produce diastolic dysfunction, such as LV hypertrophy and ischemia, are associated with reduced early diastolic filling and an augmented importance of atrial systole. It is important to recognize that such patterns can occur in patients who do not have clinically apparent diastolic dysfunction and in normals. Furthermore, a normal pattern can occur in patients who have severe diastolic dysfunction. A reduced early diastolic filling, in the absence of pulmonary congestion, indicates the loss of diastolic reserve, since the left atrium is being used as a booster pump. This pattern of diastolic filling in a patient who has symptoms of pulmonary congestion may suggest diastolic dysfunction, even if the systolic LV performance is normal. Since diastolic filling of the LV results from a complex interplay of factors, it is unlikely that a single, easily interpreted index of LV diastolic performance will ever be developed. However, the recent development of a noninvasive evaluation of the pattern of LV diastolic filling by RNA or Doppler echocardiography is an important advance. When interpreted with an understanding of the determinants of LV filling and the patient's clinical status, these noninvasive tests can contribute to the rational assessment of LV diastolic performance.