Heart failure (HF) and acute renal failure (ARF) are two very prevalent entities in our environment which impact directly and synergistically in the morbidity and mortality of our patients. ARF, when oligoanuric, often leads to water overload. It represents the precipitating core of the mechanism of acute decompensation of the HF and is associated with the worsening of symptoms, hospitalisation and death. Determining the water balance in HF can be complex and depends, largely, on the underlying pathophysiology. New biomarkers and new technologies are proving to be useful for the detection and identification of risk of acutely decompensated HF that may allow early intervention and reversal of the ARF that translates into better clinical outcomes.