Natural gas measurement today is accomplished through the use of two different classes of gas meters. These are inferential type meters, which include orifice and turbine meters, and positive displacement meters, which include diaphragm and rotary displacement meters. The inferential type meters are so-called because rather than measuring the actual volume of gas passing through them, they “infer” the volume by measuring some other aspect of the gas flow and calculating the volume based on the measurements. The positive displacement type meters are so-called because they measure the actual volume of gas displaced through them. The rotary positive displacement meter has been in existence for over 75 years. Its reliability, rangeability, long-term accuracy, and ease of installation, maintenance and testing have made this meter a favorite among gas utilities for billing purposes in industrial and commercial applications. Rotary Meters have also gained popularity in the production and transmission markets This paper will present basic operating principles of rotary gas meters, accuracy and rangeability, installation of meters, maintenance and testing, meter instrumentation, and finally, a brief glimpse at the industry trends in rotary gas metering