The important fluid dynamic principles of the flows around aerofoils, cascades of blades and the diffusing flow in radial compressor flow channels are described. The nature of decelerating flows in two-dimensional channel diffusers is explained together with their interesting flow regimes, which limit the amount of diffusion that is possible. The different nondimensional parameters used to define the fluid dynamic loading limits in compressors blade rows related to diffusion and blade loading are summarised. The primary meaning of aerodynamic loading is that there is a difference in pressure between the surfaces of a blade profile. This pressure difference leads to the forces and the torque needed to drive an impeller and to change the direction of the flow. The pressure difference between the adjacent blades may be high even if the work coefficient or pressure rise of the stage is moderate, simply by dint of having few blades. The second meaning is that the stage is designed for a particularly high work coefficient. The third meaning is related to the amount of diffusion in the flow between the inlet and the outlet of a cascade or a diffuser vane.