ESDU 02008 introduces and illustrates the time-averaged flow patterns for rectangular planform cavities mounted in a flat plate with a uniform external flow and a turbulent boundary layer@ with some reference to laminar flow where necessary. The information presented is based on a wide-ranging examination of the very extensive literature available on this subject. The flow and aero-acoustic phenomena are described largely using available wind-tunnel test data@ supported by the results of Computational Fluid Dynamics calculations where possible. The three main flow regimes@ traditionally termed open@ closed and transitional@ and the methods for their determination for subsonic and supersonic speeds@ conventionally based on the flow pattern and the characteristic shape of the mean floor pressure distribution on the cavity centre line@ are discussed. The three-dimensional effects away from the centre line@ especially near the cavity walls and corners are reviewed. Sketches of the complex three-dimensional flow structures are postulated. A summary of areas in which further research would advance the understanding of cavity aerodynamics and aero-acoustics is given. Appendix A contains a discussion of the experimental and CFD evidence for the variations in the centre-line flow pattern for open and transitional flow that occur as length-to-depth ratio is varied. The principal changes involve switches from single to tandem vortices as length is increased and from single to multiple stacked vortices as depth is increased. Appendix B contains a short discussion of recent experimental and theoretical work using information derived from fluctuating pressure data to supplement the traditional use of the characteristic floor pressure distribution shapes to determine the boundaries between flow regimes. ESDU 02008 is one of a group of Data Items intended to act as an introduction to the aerodynamics and aero-acoustics of rectangular planform cavities. The group will consist of five parts: Part I: Time-averaged Flow B ESDU 02008 (this Data Item)@ Part II: Unsteady Flow and Aero-acoustics B ESDU 04023@ Part III: Drag and Alleviation of Unsteady Flow Effects B (Data Item to be issued)@ Part IV: Overview of CFD methods ( Data Item to be issued)@ Part V: Bibliography and Tabular Survey B (Data Item to be issued).