Over the last decade, it has become apparent that left ventricular diastolic dysfunction, independent of LV systolic dysfunction, is a significant contributor to morbidity and even mortality in critically ill patients. The current method of defining and grading diastolic dysfunction in critically ill patients is derived from the definition used in the general population. Since the loading condition, the use of inotropes, and mechanical ventilation can dynamically affect myocardial relaxation, a deeper understanding of diastology, how to assess diastolic function with an echocardiogram, and caveats on the use of the current definition is crucial. In this chapter, we detail our current understanding of the pathophysiology of diastolic dysfunction during critical illness, elucidate parameters used to define and grade left ventricular diastolic function, and caveats to their application in critically ill patients.