"The information given in ESDU 04023 concerns unsteady flow and the associated aero-acoustics of rectangular planform cavities. Following a general introduction concerning the ranges of cavity geometry over which sustained oscillations can be expected@ the three oscillation types@ i.e. fluid-dynamic@ fluid-resonant - including the special case of normal mode resonance@ and fluid-elastic@ as suggested by Rockwell and Naudascher@ are discussed together with the various mechanisms involved. The parameters that can affect the frequency and amplitude of cavity oscillations for the important fluid-dynamic and fluid-resonant types involved in aircraft applications are detailed. The physical processes involved in a length mode fluid-resonant oscillation cycle are discussed. The historical view of the oscillation process as suggested by the classical work of Rossiter@ Bilanin and Covert@ Heller and Bliss@ Rockwell and Naudascher@ with later work by Heller and Delfs@ precedes the more detailed analyses made possible by the CFD simulations of Zhang et al.@ Tam et al. and Henderson. The various ""engineering"" methods for the prediction of cavity oscillations are outlined and examined. The range of empirical and analytical methods for predicting tonal frequencies in length mode@ including the widely used modified form of Rossiter's method are presented. So is a development of East's semi-empirical method for predicting the fundamental depth mode frequency at low speeds@ based on the theoretical pressure amplification equation established by the work of Plumblee et al. The new development uses East's simple equation as a basis for the provision of a more general equation providing a fairing between the low width/length ratios covered by Plumblee et al. and the high@ quasi-two-dimensional@ values for which East's original method was derived. The scope of the available engineering methods for tonal amplitude prediction is limited to that of the Cavity Acoustics Prediction (CAP) Code for length modes and the Plumblee et al. method for depth modes. Concluding remarks include a list of those areas requiring further information to provide a firmer basis for some of the discussion in the Data Item. Appendix A contains basic information on the various measures of fluctuating pressure and their analysis. ESDU 04023 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@ Part II: Unsteady Flow and Aero-acoustics B (this Data Item)@ 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). "