Traditional office measurements of blood pressure are commonly used to initiate and monitor therapy for hypertension, but these measurements are limited in their ability to provide information from the patient's normal work or play environment and do not include data from the overnight period when the patient is asleep. Thus, much potentially important information is lost. The ambulatory blood pressure monitor offers the attractive advantage of providing multiple blood pressure measurements from a subject's normal environment during his normal activities, thereby revealing important patterns of blood pressure in health and in illness. Further, the results of ambulatory monitoring have an excellent correlation with end-organ damage and these data can be obtained in a very short time period. This review will discuss the chronobiology of blood pressure, the clinical uses of the ambulatory blood pressure monitor in health and in disease, including the patterns of blood pressure identified, correlation with end-organ damage and its uses in clinical trials of antihypertensive medications; the experience in children with this technology will also be discussed.