The results of tests are described in which simply-supported plates having steel skins and a grout filler were subjected to local penetration under both quasi-static and dynamic conditions. The influence of the grout thickness was examined in detail and the dominant deformation and failure mechanisms were identified. The results show that, in contrast to tubes of a similar construction, the energy absorbing performance of these plates is inferior to that of monolithic steel plates. The reasons for this are discussed. It is shown that despite this, the behaviour of the sandwich plates is noticeably less sensitive to projectile nose shape than monolithic steel plates, and therefore this type of plate construction may be useful, in a modified form, in situations where monolithic steel plates are vulnerable to unfavourable impact conditions. Under impact loading conditions the lower skin of the sandwich plate is shown to behave similarly to a free standing quasi-statically loaded skin and to be the dominant energy absorbing component. The grout filler under impact loading absorbs energy in proportion to its thickness.