Heart failure is one of the leading causes of hospitalizations in the United States. Concomitant and significant renal dysfunction is common in patients with heart failure. Increasingly, the syndrome of heart failure is one of cardiorenal failure, in which concomitant cardiac and renal dysfunctions exist, with each accelerating the progression of the other. One fourth of patients hospitalized for the treatment of acute decompensated heart failure will experience significant worsening of renal function, which is associated with worse outcomes. It remains unclear whether worsening renal function specifically contributes to poor outcomes or whether it is merely a marker of advanced cardiac and renal dysfunction. Diuretic resistance, with or without worsening renal function, is also common in acute decompensated heart failure, although the definition of diuretic resistance, its prevalence, and prognostic implications are less well defined. The term cardiorenal syndrome has been variably associated with cardiorenal failure, worsening renal function, and diuretic resistance but is more comprehensively defined as a state of advanced cardiorenal dysregulation manifest by one or all of these specific features. The pathophysiology of the cardiorenal syndrome is poorly understood and likely involves interrelated hemodynamic and neurohormonal mechanisms. When conventional therapy for acute decompensated heart failure fails, mechanical fluid removal via ultrafiltration, hemofiltration, or hemodialysis may be needed for refractory volume overload. While ultrafiltration can address diuretic resistance, whether ultrafiltration prevents worsening renal function or improves outcomes in patients with cardiorenal syndrome remains unclear. Evidence regarding the potential renal-preserving effects of nesiritide is mixed, and further studies on the efficacy and safety of different doses of nesiritide in heart failure therapy are warranted. Newer therapeutic agents, including vasopressin antagonists and adenosine antagonists, hold promise for the future, and clinical trials of these agents are underway.