Foams are gas filled integral structures in which the gas is finely dispersed throughout acontinuouslyconnected solid phase. The bulk density is usually substantially lower than that of the solid component, and for the foams which form the focus for this book the volume fraction of the gas phase is considerably greater than 0.5 and in most instances in excess of 0.9. Many ofthe materials encountered in every day experience, such as bread, plants and trees, structural materials for buildings, comfort materials for domestic and automotive seating, shock absorbers or car bumpers and materials for noise control, have one thing in common - the cellular nature of their physical structure. Whyare thesestructuressoimportantin the naturaland man-made world? The reasons are both technical and commercial. From a technical viewpoint cellular materials offer: 1. high specific stiffness and strength - making them suitable for structural applications; 2. closeto idealenergymanagement - hencetheir useinthermalandacoustic insulation, vibration damping, acoustic absorption and shock mitigation; and 3. comfort - hence their use for domestic and automotive seating.