Since the discovery of vulcanisation in the nineteenth century, rubber has been a major industrial product. From its inception, the use of vulcanising agents, reinforcing fillers and other additives has been a major feature of the rubber industry. Innumerable articles and texts attest to the chemist's skill in balancing the chemical and physical properties of the manufactured products. In most cases, experimenters have been concerned with how recipe changes affect the product properties while the physical processes which formed the test specimen are not considered. For the rubber processor, however, it is these mechanical operations which form the heart of his business. The equipment needed for plant- scale production requires millions of dollars of capital investment. In the highly competitive rubber industry, the ability to save two or three cents per pound of product through better design or more efficient operation of mixing equipment can make a tremendous difference in the profitability of a company. Despite the commercial importance of the process, no comprehensive analysis of rubber mixing, considered as a unit operation, is currently available. This monograph is designed to fill that gap in the arsenal available for problem solving by the production engineer or the machine designer. Mixing as a general operation may be considered as three basic processes occurring simultaneously. Simple mixing ensures that the mixture has a uniform composition throughout its bulk, at least when viewed on a scale large compared to the size of the individual particles. In the case of solids blending (Chapter 11), the particle size need not change, but the distribution of particles throughout the mixture approaches a random distribution. If the shear forces are sufficiently large, particles may fracture, as in dispersive mixing, and the polymer may flow, as in laminar mixing (Chapter 111). In both of these processes, the size of the original particles or fluid elements changes because of the mixing process. Then the properties of the mixture depend upon the size of the basic structures reached during mixing. In the case of laminar mixing, the size may be the striation thickness of a hypothetical fluid element, which is inversely related to the total shear strain. If relatively strong particles, or aggregates of particles, are present, these must be reduced in size by the action of forces generated by flow in the mixer. Then the size is the actual additive particle size. The relative balance between the importance of these three processes in determining the efficiency of mixing and the product quality depends upon the attraction between additive particles, the rubber flow properties, the geometry of the mixer and the operating conditions such as temperature, mixing time and rotor speed. The interaction of operating conditions, raw material properties and the quality of mixing can be a formidable phenomenon to analyse. However, in many cases a number of simplifying assumptions about the operation can be made.