Since their introduction to the natural gas industry in the mid-1990s, multipath ultrasonic flow meters have developed a large installed base and have become the meters of choice for a variety of reasons. While one of the initial goals of the manufacturers was to develop a meter that did not require flow calibration, the accuracy requirements of most measurement applications dictate that ultrasonic flow meters need to be flow calibrated. This paper provides an overview of the calibration process and elements that should be considered by those responsible for the calibration.