The use of Liquid Ultrasonic Meters for liquid petroleum applications such as custody transfer or allocation measurement is gaining world wide acceptance by the Oil Industry. Ultrasonic technology is well established but the use of this technology for custody transfer and allocation measurement is relatively new. Often users try to employ the same measurement practices that apply to turbine technology to the Liquid Ultrasonic. There are some similarities such as: the need for flow conditioning, upstream and downstream piping requirements but there can also be differences such as the proving technique. This paper will discuss the basics of liquid ultrasonic meter operation and performance. While proving liquid ultrasonic meters is not specifically discussed, diagnostic information available to troubleshoot meter performance in general will be presented.