In almost all process plants, flow is a parameter of prime importance, for the simple reason that most processes involve moving material from one part of the plant to another or from one piece of processing equipment to another. Each movement involves doing work on the material so as to reduce or increase its mass, alter one or several of its properties, add heat, or otherwise change its condition. In this chapter we shall learn the relationship between the volume flow and the differential head created when a restriction such as an orifice plate or a venturi section is placed in the flow stream. Although the use of meters based on this principle is diminishing, there are still a large number in operation today.