Although fluid overload is one of the most prominent features of acute heart failure (AHF), its mechanism remains challenging, due to the lack of consistent data from prospective studies. Traditionally, fluid overload was thought to be mainly the result of either increased intake of fluid and salt or non-adherence with diuretic therapy. However, recent data showed little weight change before or during an AHF event suggesting that in many cases fluid overload is caused by other mechanisms such as fluid redistribution and neurohormonal or inflammatory activation. Redistribution may be the result of a combined vascular and cardiac process reducing capacitance in the venous system (and hence increasing preload) and increasing arterial stiffness and resistance (and hence afterload). When these vascular processes occur acutely and are superimposed on reduced cardiac function; fluid is redistributed to the lungs instigating pulmonary congestion. In this paper we elaborate on this possible pathophysiological mechanism and review its potential causes and amplifiers.