Congestive heart failure caused by abnormal diastolic function is far more common than previously recognized. The term diastolic dysfunction refers to the alteration in the complex processes whose interaction determines the resistance to filling of the left ventricle (LV) in patients with preserved LV systolic function, but with the signs and symptoms of congestive heart failure. The multifactorial nature of diastolic dysfunction and the possible coincidence with systolic dysfunction render the exact definition, accurate assessment, and differential diagnosis difficult. However, diastolic dysfunction and its clinical correlate, diastolic heart failure, have emerged over the last 10 years as a separate, yet still underestimated clinical entity. Today, characterization of diastolic dysfunction is based on well-defined cutoff values of indices of LV function. These indices, obtained during cardiac catheterization or preferably during noninvasive cardiac imaging, characterize abnormal LV relaxation, filling, diastolic distensibility, and diastolic stiffness in different cardiac diseases. Since diastolic heart failure has a more benign prognosis and requires specific forms of treatment, its reliable diagnosis and differentiation from systolic impairment of LV function is of paramount importance. This review reflects clinically important pathophysiological mechanisms of diastolic dysfunction, the modern diagnostic armamentarium for the assessment of diastolic dysfunction, and the different treatment strategies for diastolic dysfunction in cardiac diseases in which diastolic heart failure frequently occurs...