The American Gas Association published Report No. 9, Measurement of Gas by Multipath Ultrasonic Meters [Ref 1] in June 1998. It is a recommended practice for using ultrasonic meters (USMs) in fiscal (custody) measurement applications. This paper reviews some of history behind the development of AGA Report No. 9 (often referred to as AGA 9), key contents and includes information on meter performance requirements, design features, testing procedures, and installation criteria. Anticipated changes that should be published in the next revision, expected to be published early in 2006, are also presented