A range of composite sandwich panels with different polymeric foam cores and face-sheets were subjected to full-scale air and underwater blast testing. The air blast panels had glass fiber reinforced polymer (GFRP) face-sheets with three different polymeric foam cores: styrene acrylonitrile (SAN), polyvinylchloride (PVC) and polymethacrylimide (PMI). The panels were subjected to 100 kg TNT equivalent charge from a stand-off of 15 m. The SAN panel had the lowest deflection and suffered from the least damage. The underwater blast panels had either a single density or graded density SAN foam core and either glass fiber reinforced polymer or carbon fiber reinforced polymer (CFRP) face-sheets. The research revealed that there is a trade-off between reduced panel deflection and damage. All the blast research that has been performed is part of a program sponsored by the Office of Naval Research (ONR).