The "Cardio-Renal Syndrome" (CRS) is a disorder of the heart and kidneys whereby acute or chronic dysfunction in one organ may induce acute or chronic dysfunction of the other. The general definition has been expanded to five subtypes reflecting the primacy of organ dysfunction and the time-frame of the syndrome: CRS type I: acute worsening of heart function (AHF-ACS) leading to kidney injury and/or dysfunction. CRS type II: chronic abnormalities in heart function (CHF-CHD) leading to kidney injury or dysfunction. CRS type III: acute worsening of kidney function (AKI) leading to heart injury and/or dysfunction. CRS type IV: chronic kidney disease (CKD) leading to heart injury, disease and/or dysfunction. CRS type V: systemic conditions leading to simultaneous injury and/or dysfunction of heart and kidney. Different pathophysiological mechanisms are involved in the combined dysfunction of heart and kidney in these five types of the syndrome.