Resistant hypertension is defined as an uncontrolled office blood pressure (BP) despite the use of at least three antihypertensive drugs, in adequate doses and combinations, preferentially including one diuretic. It is a clinical diagnosis based on office BP measurements. Ambulatory BP monitoring (ABPM) is the cornerstone in the management of patients with resistant hypertension, as it is mandatory for diagnosis, treatment, follow-up and prognosis. In relation to diagnosis, ABPM measurements have classified patients with resistant hypertension into four subgroups: true, white-coat, controlled and masked resistant hypertension. This classification largely defines the therapeutic approach and the follow-up for each group. In this way, the target of antihypertensive treatment is ambulatory BP control and not office BP control. Chronotherapy based on ABPM values might frequently lead to a more rational treatment regimen. In relation to prognosis, uncontrolled ambulatory BP levels at baseline identify a subgroup of patients with a very high cardiovascular risk profile and a significantly worse prognosis. ABPM parameters can provide a better cardiovascular risk stratification than other traditional risk factors and office BPs.