Diuretics are among the most commonly used drugs. They primarily block active reabsorption of sodium at different sites in the nephron, thereby increasing urinary losses of NaCl and H2O. This ability to induce a negative fluid balance has made these drugs particularly useful in the treatment of a variety of conditions, edematous: congestive heart failure, nephrotic syndrome, liver cirrhosis, chronic renal failure, idiopathic edema, and nonedematous states: hypertension, hypercalcemia, nephrolithiasis, and syndrome of inappropriate antidiuretic hormone secretion. The diuretics are generally divided into three major classes, which are distinguished by the sites at which they impair the sodium reabsorption: loop diuretics at the thick ascending limb of the loop of Henle, thiazide-type diuretics at the distal tubule, and potassium-sparing diuretics at the cortical collecting tubule. The loop diuretics that are generally the most potent are furosemide, torasemide, and ethacrynic acid. The thiazide-type diuretics include chlorothiazide and metolazone. Spironolactone and amiloride are potassium-sparing diuretics. Diuretics should be started at an effective single dose and given intermittently with a subsequent increase in dose or frequency of administration. As a general rule, the rate of diuresis in an edematous patient should not exceed 1 to 2kg weight loss per day. In renal failure patients, loop diuretics at a higher than normal dose are required to get the desired diuretic effect because the diuretic excretion is often limited, in part due to the retention of organic anions. The patients with liver cirrhosis are responsive to spironolactone. After the administration of diuretics, even if a net diuresis is induced, the response is short-lived as a new steady state is rapidly established because the diuretic-induced sodium losses are counterbalanced by neuro-humorally mediated increases in tubular reabsorption at nondiuretic sensitive sites. This process is called compensatory antidiuresis or diuretic tolerance. Therefore sodium restriction is important when a patient is taking loop diuretics, and the concurrent use of a thiazide diuretic can inhibit downstream NaCl reabsorption, resulting in an exaggeration of diuresis. The most common side-effects are those encountered in virtually all the effective drugs: hypovolemia, hypokalemia and potassium depletion, hyperuricemia, and metabolic alkalosis. Other side-effects include hyperglycemia, hyperlipidemia, hyperuricemia, ototoxicity and sexual dysfunction. In addition, diuretics have the potential to increase the toxicity of several other agents. Nonsteroidal antiinflammatory drugs may antagonize the natriuretic effects of diuretics. The combination of potassium-sparing diuretics and angiotensin-converting enzyme inhibitors or angiotensin II receptor blockers may result in severe hyperkalemia.