Density reduction of cellular materials can be directly converted into cost savings and consequently commercially produced foams are being driven towards their lowest possible density for a given application. However, as densities become even lower, the control and optimization of the cellular structure and physical properties becomes more complex and a better understanding of the fundamental relationships between composition, cellular structure, matrix morphology and the physical properties is required. To facilitate this understanding, a world-wide group of authors from industry and academia at the forefront of research have been brought together. The fundamental basis of the formation and the physical behaviour of low density cellular polymers are described and analyzed and they report, explain and quantify, using empirical data, phenomenological modelling and mathematical analysis; the current understanding of fundamental issues concerning foam formation and physical behaviour. Current model representations of physical behaviour are critically reviewed and model predictions compared with experimental data. Unlike other books on polymeric foams, which concentrate on the chemistry and process technology of foams, it focuses on the essential material science issues of the quantification and mathematical modelling of physical behaviour whether it be mechanical, pneumatic, thermal or acoustic. In addressing fundamental issues concerning the physics of low density cellular plastics, the reader's attention is directed to the many possibilities in foam development which have potential for commercial exploitations. This book should be of interest to senior undergraduates, graduates and researchers working in the plastics field; those working in the plastics industry.