This Data Item introduces the subject of flow in rotating machinery and provides guidance and techniques for modelling specific flow applications involving discs, cylinders and cavities, predominantly in steady-state, stable flow conditions. An interest in the fluid flow associated with rotating machinery may stem from the need to know the mechanical losses and flow structure in electric motors, centrifugal compressors or rotary pumps. Emphasis within this Data Item has been placed on presenting correlations based on analytical, experimental and computational fluid dynamics (CFD) models for parameters such as mass flow, boundary layer thickness, swirl speed, pressure distribution and moment, used in parametric design studies and optimisation. This Data Item includes consideration of laminar, transitional and turbulent flow for the case of a disc rotating in a fluid. This case is used as the basis for extension to more complex rotating disc applications such as those involving the disc rotating adjacent to a stationary disc with a shroud and superposed supply of fluid, i.e. rotor-stator wheelspace systems. The cases of a rotating cylinder in a fluid and of an annulus where one or both of the cylinders are rotating are also considered. A common configuration for rotating cavities where two adjacent discs are co-rotating is briefly introduced.