There is no doubt that patients with high blood pressure (BP) are at higher cardiovascular and death risk than those subjects whose BP levels are below the admitted normal threshold. However, most of the epidemiological surveys show that BP is uncontrolled in more than fifty percent of hypertensive subjects. There are several reasons that can justify this lack of hypertension control, some of them depending on the patient, such as therapeutic adherence, or some related to the doctor, due to therapeutic inertia or reluctance to increment the number and doses of antihypertensive drugs. Sometimes the efficacy or adverse effects related to the antihypertensive drugs underlie the uncontrolled hypertension. And, finally, there are some clinical conditions that are associated with difficult-to-control hypertension. Among them, comorbidities such as diabetes, obesity, obstructive sleep apnoea syndrome or chronic kidney disease, but also drug-related hypertension or resistant hypertension. In this article we review the epidemiology and the conditions which are related to poorly controlled hypertension and that can explain why hypertension may become difficult-to-treat.