Mixtures are difficult to define within the range of materials employed in the rubber and plastic industries. A composite article such as a tire is a mixture of wire (metal), textile cord (organic fiber), and rubber compounds. The rubber compound itself is a mixture of elastomer, filler, and, usually, extender. Moreover, the elastomer may be a mixture of two or more rubbers. On a smaller scale, the elastomer may contain ‘blocky’ segments or crystalline portions along the polymer chains which act as ‘reinforcing’ agents or stiffeners. These may be similar to the repeat units of the elastomeric part, e.g., isotactic—atactic block copolymers of poly(propylene) or poly(propylene oxide); or different from it, as in the stereo block styrene—butadiene thermoplastic copolymers. Two types of elastomer may of course, be copolymerized or one elastomeric type may be grafted onto another. Furthermore, certain catalysts could cause simultaneous homopolymerization of two monomers. All the foregoing systems are different facets of the broad concept of blends, but some selection is obviously needed. In this review, most emphasis will be placed on elastomer—elastomer blends including their preparation from the constituent elastomers. Reference will be made to elastomer—plastic blends prepared from separate materials and by block/graft type reactions. Blends of one plastic with another lie outside the scope of this review.