On first thought, the properties controlling the behavior of high molecular weight substances would appear to be so overwhelmingly numerous as to defy consolidation and simplification. However, upon analysis, it can be seen clearly that they all reduce to areas of consideration which make up a relatively small group. For elastomers, such a grouping might be as follows. Size Shape Order and Disorder Composition Crosslinked Network Response to Deformation Degradative Reactions This reviewer does not intend to imply that each area can be considered mutually exclusive of any of the others. There is ample evidence of considerable interaction between these areas. Moreover, it is not intended to imply that the approach to elastomeric problems is utterly simple. The complexity within any one of the areas mentioned is well known from the literature on polymers. However, it is suggested that a grouping or classification such as given here can assure a greater clarity of concept and objectivity of analysis and experiment in approaching otherwise obtuse and complicated polymer problems. As each new polymer material comes under research scrutiny in this sense, the general “laws” governing polymer behavior are clarified and extended. This is a result of the fact that each new material usually provides features which invite and encourage research, particularly where uncertainty exists. The ease of butyl type polymers will be reviewed in this sense with two purposes in mind: (1) to define and correlate the knowledge derived from butyl research which has contributed to a better understanding of the principles of polymers, and (2) to suggest (often by inference) areas where potentially fruitful research in the same direction is still possible. This review will consider butyl polymers beyond the polymerization stage. Except where necessary to develop the proposed objectives, the chemistry of polymerization will not be considered. A number of references are recommended to the reader interested in the polymerization aspects. No attempt will be made in this review to consider butyl end uses or the compounding and technological information associated therewith. Some exception to this restriction will be made when the particular property under consideration warrants it. Rather, this review will attempt to show how a development of the basic knowledge of polymer behavior can be put to use in the solution of technological problems.