An article on crystallization in natural and synthetic rubbers has been prepared by Leonard A. Wood of the Bureau for publication as a chapter in the book entitled "Advances in Colloid Science" now being edited by H. Mark and G. S. Whitny. As Dr. Wood explains, crystallization may be studied by a number of different methods. These include volume changes, thermal measurements, X-ray diffraction, optical double refraction, light transmission, and a study of mechanical properties such as modulus and hardness, or those evident in flow. Crystallization may occur in natural rubber, balata, gutta percha, chloroprene polymers (Neoprene), organic polysulfides (Thiokol), isobutene polymers (Vistanex and Butyl rubber), polyethylene, and in vinylidene chloride polymers and copolymers (Saran). Experimental results, together with a certain amount of interpretation, are presented regarding rate of crystallization, melting of crystals, change of melting range with temperature of crystallization re-crystallization, fraction of material in the crystalline state, and the effects of vulcanization on vulcanization on crystallization. While a certain emphasis is placed on work carried out at the Bureau, the chapter is intended as a summary of the field and contains over ~4 o references to the literature.