Renal dysfunction (RD) is a frequent comorbid condition and a major determinant of outcomes in patients with heart failure (HF). It is likely that the etiology of RD in patients with HF is much more complex than we first thought and represents a matrix of independent, albeit interacting, pathophysiological pathways with effects on both the kidney and the heart that share a common denominator: aging and inflammation. Renal dysfunction in HF has been attributed, among others, to biochemical, hormonal, and hemodynamic factors, coupled with pharmacological interventions. Regardless of the cause, the development of RD or worsening renal function is common in patients with HF, and is associated with increased morbidity and mortality. There is increasing evidence, however, that transient increases in creatinine in the setting of acute HF are not prognostically important, whereas persistent deterioration does portend a higher mortality in this patient population. In addition, congestion seems to play an important role in the course of renal deterioration, and the combination of congestion and worsening renal function is the most significant clinical prognosticator in HF patients. This review aims to provide an update on the epidemiology and prognostic significance of RD in HF patients, in both the acute and the chronic setting.