ESDU 02009 provides methods for the estimation of averaged radiation efficiencies of orthotropic@ rectangular plates which are simply supported at all edges and surrounded by an infinite baffle. Isotropic plates are also included as a special case of orthotropic plates. The method used is approximate and based on an asymptotic approach. The background to the subject and the nature of orthotropic plates are explained. The document applies to plates made of naturally orthotropic materials@ such as wood. It can also be used with equivalent orthotropic plates@ such as ribbed@ corrugated or honeycomb panels. Plates of composite materials@ such as carbon fibres in epoxy@ are within the scope of the document. In order to estimate the sound radiation efficiency the dimensions of the plate@ the density and the bending stiffnesses are required. Two methods are available for the estimation of radiation efficiencies. The first method is based on design curves presented in the document and in the second method computer programs are used. Each design curve is able to apply for a range of non-dimensional parameters which influence the radiation efficiency. Use of the design curves gives results which are accurate to within 2 dB. The results from the computer programs are more accurate than the results deduced from the design curves. The computer programs provide output in one-third or one-sixth octave bands. Applications are described for wooden and composite plates@ corrugated and ribbed plates and honeycombs. Two appendices are included. The first gives expressions for bending stiffnesses for different equivalent orthotropic plates. In the second appendix methods of deriving the bending stiffnesses of plates of composite materials are described with examples which apply to a single layer (Kevlar in epoxy) and to a hybrid sandwich with five layers. Two computer programs@ ESDUpacs A0209 and B0209 are provided with this Item.