Gas supply is a part of the critical infrastructure. A large part of the industry, power plants as well as utilities generating heat rely on its availability. This presentation sheds light on the necessity of the measurement, as well as the diverse metering system. To illustrate the complexity of each of the measurement devices in the system, the technology of volumetric flow measurement using ultrasound is introduced exemplary. Consequences of the accuracy requirements of the authorities on the meter design are discussed. Limitations of ultrasonic meters with respect to flow velocity, gas composition and others, are presented.