Cardiovascular disease is the leading cause of death in patients with chronic kidney disease. Heart failure may lead to acute kidney injury and vice versa. Chronic kidney disease may affect the clinical outcomes in patients with cardiovascular disorders. Renal impairment with any degree of albuminuria has been increasingly recognized as an independent risk factor for cardiovascular events and heart failure hospitalizations, while chronic heart failure may cause chronic kidney disease. The bidirectional nature of these disorders contributes to the complexity and the composite definitions of cardiorenal syndromes. However, the most important clinical trials in heart failure tend to exclude patients with significant renal dysfunction. The mechanisms whereby renal insufficiency worsens the outcome in heart failure are not known, and several pathways could contribute to the ‘‘vicious heart/kidney circle.’’ Traditionally, renal impairment has been attributed to the renal hypoperfusion due to reduced cardiac output and decreased systemic pressure. The hypovolemia leads to sympathetic activity, increased renin-angiotensin aldosterone pathway, and arginine-vasopressin release. These mechanisms cause fluid and sodium retention, peripheral vasoconstriction, and volume overload. Therapy to improve renal dysfunction, reduce neurohormonal activation and ameliorate renal blood flow could lead to a reduction in mortality and hospitalization in patients with cardiorenal syndrome.