: Adherence to treatment is now well recognized as a crucial key in the effectiveness of antihypertensive drugs; however, it is often overlooked in the management of hypertension because methodology to assess it is partly unreliable and limits its use in clinical practice. The available evidence suggests that nonadherence is highly prevalent in a chronic asymptomatic condition such as hypertension. It may undermine benefits expected from antihypertensive agents and therefore, may negatively impact cardiovascular, cerebrovascular and renal outcomes. In this review, we discuss the methodological issues related to the measurement of drug adherence in a research setting and clinical practice, the prevalence and the impact of drug nonadherence on blood pressure control and thus in apparent resistant hypertension, and on cardiovascular, cerebrovascular and renal outcomes.