The successful use of diuretics depends upon a knowledge of their mode of action, their potency, and their side-effects. The four basic groups of diuretics which cover most situations are: (i) the thiazides, (ii) ethacrynic acid or frusemide, (iii) spironolactone or triamterene, (iv) mannitol.The thiazides have a moderate diuretic action and are useful in any oedematous state where a gradual removal of salt and water is required. Mild to moderate cardiac, cirrhotic, and renal oedema respond well to thiazide administration, and the thiazides are useful in systemic hypertension, diabetes insipidus, and in the treatment of hypertension and oedema associated with pregnancy.