The heart is a complex and sophisticated pump that cycles between two phases: diastole, during which a compliant chamber (ventricle) allows the blood to fill from a reservoir chamber (atrium) of low pressure, and systole, during which a stiff chamber with rapidly rising pressure ejects the blood into an arterial circuit of high pressure. However, the systolic and diastolic cycles are not dichotomous. They have complex interactions with interrelated segments of the cardiac cycle. Although the entity of “diastolic heart failure with preserved systolic function” has been applied in adult patients, a discrete diagnosis of systolic and diastolic heart failure may be difficult to apply in pediatric patients. Advances in echocardiography have helped decipher the morphologic and physiologic expression of congenital and acquired heart disease and have increased our understanding the diastolic function and dysfunction. The evolving concept of systolic and diastolic heart failure is helping us develop a strategy for its management in pediatric patients with complex heart diseases.