This book describes how to calibrate, measure and specify ultrasonic flow meters to ensure that the legal metrology requirements are met in custody transfer and fiscal applications. This book is intended for metrologists who are in charge of specifying ultrasonic flow meters, calibrating or accurately measuring water, oil and natural gas flowrate in custody transfer and fiscal measurement applications, and require a deep knowledge of the measurement principles and the results that can be achieved, so that the legal metrology requirements can be met.