CLASSICAL FEATURES AND SCREENING: The classical features of primary aldosteronism-hypertension, hypokalemia and metabolic alkalosis-were first described by J. Conn in the midfifties of the last century. The classical form of primary aldosteronism is a rare disease with prevalence rates of 0.1-0.5% within the hypertensive population. The normokalemic variant of primary aldosteronism seems to be much more frequent (5-13%). Although a validated and standardized diagnostic protocol for this entity is still missing recent studies established the aldosterone to renin ratio as a useful screening test. To increase diagnostic sensitivity and specificity of the ratio aldosterone should be added as second screening criterion (sensitivity and specificity about 90%). Dynamic confirmatory testing proving autonomous aldosterone secretion is required to verify the diagnosis in case of a positive screening test. A simple confirmatory test is the salt loading test. Alternatively, the fludrocortisone-suppression-test, the Captopril-challenge- test or the daily exretion rate of aldosterone-18-glucuronide and tetrahydroaldosterone in urine can be used. In case of proven primary aldosteronism further diagnostic evaluation (e. g. CT scanning, postural-test and in case of discrepancy adrenal vein catheterization) is mandatory to differentiate the most common forms of primary aldosteronism, aldosterone producing adenoma and idiopathic hyperaldosteronism. Since many patients with primary aldosteronism can be cured by surgery and missing the diagnosis often leads to significant end-organ damage it is important to evaluate hypertensive patients with therapy-resistant hypertension for primary aldosteronism.