As ambulatory blood pressure measurement becomes more widely accepted in hypertension research and in the clinical management of high blood pressure, the number of devices available on the market has increased considerably, reflecting the clinical demand. These devices are expensive, both in terms of capital and running costs. As we rely increasingly on data produced by ambulatory systems, it becomes increasingly important that they be shown to be accurate. Initially protocols for the validation of ambulatory devices were designed on an ad hoc basis, so that many studies were inadequately designed and comparison of data between studies was usually impossible. The American National Standard published by the Association for the Advancement of Medical Instrumentation (AAMI) remedied this situation in part, and the recent publication of the British Hypertension Society (BHS) protocol for the validation of ambulatory systems has further advanced the demand for accuracy. The BHS protocol includes most of the AMMI standard recommendations, but in addition there are sections on observer training, in-use assessment and inter-device variability. In addition, performance characteristics, computer facilities and details on such practical matters as cost and maintenance are sought. Finally, the BHS protocol provides a grading system of validation which allows comparisons between devices and studies.