The preceding review suggests that for structural foam formation the gas may be introduced directly during processing or may be formed as the result of breakdown of chemical blowing agents such as azodicarbonamide. The choice of the chemical blowing agent depends on the polymer processing temperature and the end-use of the product. The ideal chemical blowing agent should decompose over a definite and short range of temperature. When chemical blowing agents are used for foam production, the optimum processing conditions are that decomposition of blowing agent should be incomplete in the extruder and begin in the metering section of the extruder where high pressures will dissolve the evolved gases in the polymer. The dissolved gas becomes supersaturated and begins to form bubbles when it emerges from the die into the low pressure. The bubble formation occurs at sites nucleated by the decomposition of the remaining blowing agent. This optimum foaming condition can be accomplished by extruding at high shear rates through a die. The effect of bubbles on the tensile properties and drawing behavior of polymer is very different according to polymer structure. Generally, physical properties exhibit higher values in fine-celled high density foam than large-celled low density foams. More information is needed on the influence of nucleating agents on foam properties. Suitable tests to measure foam quality quantitatively are needed.