This review has highlighted some of the major problems in polymer degradation. It will be apparent that the degradation of commercial rubbers and plastics is exceedingly complex, since processing and small quantities of impurities or additives can interact and totally change the behavior of the substrate. It is not sufficient to examine the degradation of pure materials. The pure polymer may be studied as a base line but in order to obtain the full story it must also be studied in presence of known impurities or additives added singly and also in combination. This systematic approach is laborious and seldom used. The need for more realistic tests is now being recognized. Outdoor exposure combines the effects of uv light, heat, rain, ozone, nitrogen dioxide, and sulfur dioxide. A laboratory test should take all such elements into account. If a part is to be exposed to oil and air in the presence of heat, as it may be in an automobile, laboratory tests must examine the combined effects of these. The literature contains a wealth of information on the degradation of model compounds and pure materials, and copious empirical information on the behavior of commercial materials. The explanation of what happens in practice and, especially, the production of commercial polymers which give close to ideal behavior, is much more elusive.