The increasing use of synthetic rubbers of different compositions and properties has necessitated the development of methods for characterising each individual member. Until recently only one rubber, i.e. natural rubber, was likely to be encountered, and detailed methods for its analysis have already been laid down by the British Standards Institution and by the American Society for Testing Materials. The identification of natural rubber rarely presented any problem, since there were few, if any, materials resembling it in either appearance or general behaviour. Nowadays, however, the situation has changed, and problems frequently arise which demand the rapid identification of synthetic rubbers, either alone or in admixture with other rubbers, including the natural product. The purpose of the present chapter is to survey the whole field of the identification and estimation of rubbers as outlined above and to provide a manual of methods adopted as the result of trials carried out by a number of independent laboratories. The inclusion of a method in this publication must not be taken to imply that it has received official approval. Since the publication of Memorandum No. U.9 in January 1944, much further experience has been gained in the analysis of natural and synthetic rubbers, and the opportunity has therefore been taken in preparing this edition to revise the whole of the text.