Publisher Summary This chapter describes various laws that are recognized and met before flow measurement is attempted. Some of these laws explain what happens when fluid flows in a pipeline, and these in turn explain what happens in a flowing stream as it goes through a meter. Fluid friction law states that energy is required to overcome friction to move fluid from one point to another. The main energy concerns are the potential and kinetic energies, the others are neither important nor change position from one point to another. The chapter discusses the Reynolds number, which is a useful tool in relating how a meter will react to a variation in fluids from gases to liquids. Calculation of Reynolds number defines the flow velocity pattern and approximate limits of the meter's application. The real gas law corrects for the fact that gases do not follow the ideal law at conditions for high pressure and low pressure. Emperically derived values for various gases are available in industry standards or are predicted by correlations based on their critical temperature and pressure. In the concluding part of the chapter, fundamental flow equation is presented.