Impaired pump function in the patient with congestive heart failure can arise from both contraction (inotropic) and relaxation (lusitropic) abnormalities. Because energy is required for both contraction and relaxation, evidence that the failing heart is in an energy-starved state may explain, at least in part, these functional abnormalities. The relative contributions of inotropic and lusitropic abnormalities to clinical disability differ in individual patients with heart failure, so that therapy of heart failure must be tailored to the specific pathophysiology that operates in each patient.