Cardiovascular morbidity and mortality resulting from congestive heart failure are major concerns for the critical care physician. Although heart failure is commonly associated with impaired systolic function, in up to one half of cases, heart failure occurs exclusively on the basis of an impairment of diastolic function. Diastole is the summation of processes by which the heart loses its ability to generate force and shorten and returns to its precontractile state. The two principal processes responsible for diastole are relaxation and passive pressure–volume properties of the ventricle. Echocardiography provides a comprehensive, noninvasive evaluation of diastolic filling of the ventricle, myocardial relaxation, and ventricular stiffness; the information obtained by echocardiography has prognostic value and is a guide to proper therapy. This article reviews the physiology of diastole, the pathogenesis of diastolic heart failure, and the diagnosis of diastolic dysfunction, with a focus on the diagnostic utility of echocardiography and an emphasis on those areas of greatest interest to the critical care physician.