This paper discusses fundamental principles of ultrasonic gas flow meters used for measurement of natural gas and the available basic diagnostic capability to assess meter operation and performance. 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 manufacturer’s transit time ultrasonic flow meters however, these examples provided, particularly with respect to some diagnostic features, are based on the Daniel SeniorSonic ultrasonic flow meter. Advanced diagnostic data, in conjunction with gas composition, pressure and temperature, that provides diagnostic benefits beyond that of other primary measurement devices is outside the scope of this paper, though these topics will be covered in the companion paper, Ultrasonic Meter Diagnostics – Advanced.