The requirements for minimum weight structural designs in aerospace applications of sandwich plates inherently involve considerations ofsituations in which the assumptions leading to geometrically linear formulations of structural analysis problems are not valid. In order to predict correctly the sandwich behaviour, it has frequently become necessary to employ more exact equilibrium equation [I ] or strain-displacement relationships [2]: i.e., to consider the non-linear behaviour of the sandwich materials. However, the small deflection theories for plates, whether or not subjected to transverse shear effects, are based on the assumptions that the average transverse deflection is less than the plate thickness. This limitation is fairly well accepted for plates rigid in shear but the limits for a sandwich plate are not necessarily the same and will depend on the shear stiffness of the plate. As G -+ 0 then w --~ ]12 and as G -+ co then w -,'-(2hl + h2). Reissner [3] has derived an approximate expression for estimating the deflection of a simply supported isotropic square sandwich plate for which linear theory is valid: