Ultrasonic meters have been used in gas custody transfer measurement worldwide for over 25 years with varying degrees of success. Initial attempts proved unstable and maintenance intensive, this was contrary to the initial expectations which foresaw a device with little or no obstruction, limitless turndown and little to no required maintenance. The advent of higher speed, more robust electronics enabled the use of digital signal processing which eliminated the need for analog threshold levels and the constant problem of peak skipping and lost timing. The improved electronics also enabled the implementation of large internal logs, advanced diagnostics, improved communications and overall stability, all of which increased user confidence in the ultrasonic technology. Today meters have the ability to store years of audit, alarm and operational data, communicate via Ethernet, locally and remotely, continually monitor diagnostic parameters and alarming when values exceed preset limits. This paper will briefly cover GUSM history, look at USM transit time basics, path configurations, meter construction, installation requirements and trends to look for in the future.