This chapter provides an introduction to foams and foam formation. Foams may be flexible or rigid, depending upon whether their glass-transition temperatures are below or above room temperature, which, in turn, depends upon their chemical composition, degree of crystallanity, and degree of crosslinking. Intermediate between flexible and rigid foams are semi-rigid or semi-flexible foams. The cell ... [Show full abstract] geometry—that is, open vs. closed cell, size and shape, greatly affect the foam properties. Thus, closed-cell foams are most suitable for thermal insulation, while open-cell foams are best for acoustical insulation. The preparation of polymeric foam involves the formation of gas bubbles in a liquid system, followed by the growth and stabilization of these bubbles as the viscosity of the liquid polymer increases, resulting ultimately in the solidification of the cellular resin matrix. Foams may be prepared by either one of the two fundamental methods. In one method, a gas, such as air or nitrogen, is dispersed in a continuous liquid phase to yield a colloidal system with the gas as the dispersed phase. In the other method, the gas is generated within the liquid phase and appears as separate bubbles dispersed in the liquid phase. The gas can be the result of a specific gas generating reaction such as the formation of carbon dioxide when isocyanate reacts with water in the formation of water-blown flexible or rigid urethane foams. Gas can also be generated by volatilization of a low-boiling solvent in the dispersed phase when an exothermic reaction takes places.