Secondary hypertension affects only 5-10 % of hypertensive patients. Screening is expensive and time-consuming and should be performed only in patients for whom there is a high clinical suspicion of secondary hypertension. Clinical signs of secondary forms of hypertension are new-onset hypertension in patients without other risk factors (i.e., family history, obesity, etc.), sudden increase of blood pressure (BP) in a previously stable patient, increased BP in prepubertal children, resistant hypertension, and severe hypertension or hypertensive emergencies. In adults, renal parenchymal and vascular diseases as well as obstructive sleep apnea are the most common causes of secondary hypertension. Medication-induced hypertension and non-adherence to medication have to be ruled out. Of the endocrine causes associated with hypertension, primary aldosteronism is the most common. Other endocrine causes of hypertension such as thyroid disease (hypo- or hyperthyroidism), hypercortisolism (Cushing's syndrome), hyperparathyroidism, and pheochromocytoma are rare. Monogenetic forms of hypertension are mostly of tubular origin and associated with alterations in mineralocorticoid handling or signaling. Rare causes of hypertension also include inflammatory vascular disease. Acute forms of vasculitis may present as "malignant" hypertension with associated thrombotic microangiopathy and organ damage/failure. It is important to diagnose these rare forms of hypertension in order to prevent acute organ damage in these patients or unnecessary invasive treatment strategies.