The âaudiometerâ*, a generic term perhaps preferable to âradiometerâ given its applications equally to TV as for radio audience measurement, offers the marketplace a major technological advance in the measurement of broadcast audiences. The Radiocontrol Watch is used in Switzerland for national radio audience measurement. In the United States Arbitron has mounted its Philadelphia 300 Panel. This paper reviews the critical issue of the validity of these new systems. Do they actually measure what they purport to measure? Do people actually wear the devices from first thing in the morning to last thing at night? How exposed are they to false negatives and false positives: failing to pick up media exposure that has taken place, crediting TV/radio viewing/listening when there was none? These are the early days, but the early evidence from Arbitron and Radiocontrol looks most promising.