Measuring and controlling fluid flow is important to avoid hazardous process conditions, to monitor raw material consumption and to quantify production rates. Closing a mass balance around a reactor or a unit operation is necessary to derive kinetics, trouble shoot and to optimize the process, all of which depend on measuring flow rates accurately, including liquids, gases, vapors, suspensions, slurries, and even solids. Pneumatic conveying is an industrial example where solids flow must be metered. Bernoulli (1738) pioneered the modern concept of fluid dynamics that is based on the principle of the conservation of energy. Reynolds identified the conditions in which flow in pipes is laminar or turbulent and proposed a non-dimensional relationship. As many as 33 distinct flow meter technologies assess fluid flow including direct methods like collecting and weighing liquids in bucket to indirect methods like measuring pressure drop. Differential pressure instruments are easy to construct and install but vortex meters, Coriolis meters, and ultrasonic meters are supplanting these technologies because they are more accurate and reliable.