The discovery of four major water channels in the kidney, namely aquaporins (AQP) 1, 2, 3 and 4, has allowed a substantial increase in our understanding of renal water regulation in health and disease. This review discusses the renal aquaporin water channels in the urinary dilution and concentrating defects in cardiac failure, cirrhosis, syndrome of inappropriate hormone secretion, pregnancy, hypothyroidism, isolated glucocorticoid deficiency, isolated mineralocorticoid deficiency, primary polydipsia, acquired and genetic nephrogenic diabetes insipidus.