This paper discusses fundamental principles of ultrasonic gas flow meters used for measurement of natural gas. A review of an ultrasonic meters operation and the equations used to determine actual volumetric flow is presented. The ultrasonic flow meters diagnostic capability will also be briefly presented. Further, diagnostic data, in conjunction with gas composition, pressure and temperature, will be reviewed to show how this technology provides diagnostic benefits beyond that of other primary measurement devices. The basic requirements for obtaining good meter performance, when installed in the field, will be reviewed. Most of this information can be generalized to other manufacturers transit time ultrasonic flow meters however, these examples provided, particularly with respect to some diagnostic features, are based on the Daniel Senior Sonic ultrasonic flow meter