Resistant hypertension is defined as blood pressure that remains uncontrolled despite using at least three antihypertensive medications in effective doses, ideally including a diuretic. Stricter blood pressure goals, higher obesity rates, older age, and increased use of certain exogenous substances are related to an increasing prevalence of resistant hypertension. The evaluation of patients with resistant hypertension focuses on identifying contributing factors and secondary causes of hypertension including hyperaldosteronism, obstructive sleep apnea, renal artery stenosis, and pheochromocytoma. Hyperaldosteronism is now recognized as the most common secondary cause and all patients with resistant hypertension should be screened with a plasma aldosterone-renin ratio even if the serum potassium level is normal. Treatment includes reversal of contributing factors, appropriate treatment of secondary causes, and use of effective multidrug regimens. Recent studies indicate that the addition of spironolactone to standard treatment regimens induces significant blood pressure reduction in patients with resistant hypertension.