In many books on radial turbocompressors, the nondimensional parameter known as specific speed is often used to categorise a particular type of design. The specific speed and specific diameter are two alternative dimensionless coefficients based on the same data which are used define the flow coefficient and the head coefficients. In their definition, both the flow coefficient and the head coefficient are included. The specific speed alone is often used to specify a particular type of design of radial compressor as, broadly speaking, an optimum specific speed can be defined, rather like an optimum flow coefficient. This optimum is often presented in the form of a Cordier diagram. Both parameters are essentially incompressible in nature and are often used for hydraulic machines and in pump and ventilator design. In this chapter, the background to these parameters is described. This discussion gives clear guidance that the flow coefficient, work coefficient and tip-speed Mach number are more useful for radial turbocompressors than specific speed and specific diameter.