Use of ambulatory blood pressure monitoring (ABPM) techniques has revealed that blood pressure is characterized by a considerable degree of variability over a 24 h period as a result, not only of the well-known fluctuations that occur between wakefulness and sleep, but also of the minute-to-minute changes induced by a variety of behavioural conditions. The degree of these variations is also influenced by neural mechanisms responsible for cardiovascular regulation, such as the arterial baroreflex. Blood pressure variability increases with age and blood pressure values, and its magnitude has been shown to correlate independently with the target-organ damage of hypertension. This has stimulated both the development of antihypertensive drugs able to reduce blood pressure homogeneously over 24 h, and recent proposals to develop more accurate indices, such as the smoothness index, to quantify the distribution of the antihypertensive effect over the entire day and night. Despite the important information that ABPM can provide concerning daily-life blood pressure variations and their modification by treatment, international guidelines suggest that it should not yet be used routinely in daily practice, but rather reserved for selected patients.