Radio controlled clocks represent a true revolution in timekeeping. Clocks that synchronize to NIST radio station WWVB now number in the millions in the United States, and new sales records are being established every year. As a result, many of us are now accustomed to having clocks in our homes, offices, and on our wrists that always display the correct time and that never require adjustment. This NIST Recommended Practice Guide was written to provide guidance to both manufacturers and consumers of radio controlled clocks. Through voluntary compliance with the recommended practices listed here, manufacturers can benefit by continuing to develop more reliable and usable radio controlled products, increasing both consumer confidence and sales. Consumers can benefit by using this guide to help them select and purchase radio controlled clock products, to learn how the products work, and to help troubleshoot reception problems.