More than one million hospitalizations occur annually in the US because of heart failure (HF) decompensation caused by fluid overload. Congestion contributes to HF progression and mortality. Apart from intrinsic renal insufficiency, venous congestion, rather than a reduced cardiac output, may be the primary hemodynamic factor driving worsening renal function in patients with acutely decompensated HF. According to data from large national registries, approximately 40% of hospitalized HF patients are discharged with unresolved congestion, which may contribute to unacceptably high rehospitalization rates. Although diuretics reduce the symptoms and signs of fluid overload, their effectiveness is reduced by excess salt intake, underlying chronic kidney disease, renal adaptation to their action and neurohormonal activation. In addition, the production of hypotonic urine limits the effectiveness of loop diuretics in reducing total body sodium. Ultrafiltration is the mechanical removal of fluid from the vasculature. Hydrostatic pressure is applied to blood across a semipermeable membrane to separate isotonic plasma water from blood. Because solutes in blood freely cross the semipermeable membrane, large amounts of fluid can be removed at the discretion of the treating physician without affecting any change in the serum concentration of electrolytes and other solutes. Ultrafiltration has been used to relieve congestion in patients with HF for almost four decades. In contrast to the adverse physiological consequences of loop diuretics, numerous studies have demonstrated favorable responses to ultrafiltration. Such studies have shown that removal of large amounts of isotonic fluid relieves symptoms of congestion, improves exercise capacity, improves cardiac filling pressures, restores diuretic responsiveness in patients with diuretic resistance, and has a favorable effect on pulmonary function, ventilatory efficiency, and neurohormonal activation. Ultrafiltration is the only fluid removal strategy shown to improve outcomes in randomized controlled trials of patients hospitalized with decompensated HF.