Secondary hypertension may be defined as a type of hypertension (i.e. blood pressure >140/90 mmHg) with an underlying, potentially correctable cause. Secondary hypertension should be particularly considered in (1) young patients without a family history of arterial hypertension, (2) patients with resistant hypertension, and (3) late onset of hypertension. In addition to the medical history, a secondary aetiology may be suspected in the presence of symptoms (e.g. flushing and sweating suggestive of phaeochromocytoma), clinical findings (e.g. a renal bruit suggestive of renal artery stenosis), or laboratory abnormalities (e.g. hypokalaemia suggestive of hyperaldosteronism). Approximately 5% of adults with hypertension have a secondary cause. The prevalence of secondary hypertension and the most common aetiologies vary by age group. This chapter aims to summarize the principal causes of secondary hypertension, how these may be diagnosed and their specific treatments.