This paper provides a scientific treatment for mixing of rubber with reinforcing fillers. The primary attention is given to the viscoelastic nature of rubber. It is addressed to the practitioners of rubber processing, who have hands-on experience but lack familiarity with viscoelasticity. In the first part we provide a sketch of the major events: (1) polymerization, (2) resulting molecular architecture, (3) mechanical property of gum rubber, (4) mill processability, (5) processing with the internal mixer, and (6) compounding rubber with carbon black. The focal point of this paper is to relate the mill processability to the elongation and failure behavior of gum rubber. Viscous and elastic contributions to this relation are described. This relation provides the starting point for introducing scientific treatment for rubber mixing. Mechanisms of incorporation and dispersion of carbon black are also discussed. The presentation follows in an order of description on the behavior of gum rubber, that of carbon black, the interaction between rubber and carbon black, and the interaction between the material and the internal mixer. In the second part a method of constructing viscoelastic master-curves from the tensile stress-strain data is presented. Such a master curve serves as a useful means of characterizing gum rubber in relating polymerization, resulting molecular architecture and processability. The milling and mixing of rubber with filler are achieved with fracture; the consequence of fracture is a generation of supermolecular flow units. The mechanism of fracture involves the pull out of chain ends through entanglements. The high concentration of the chain ends on the surface gives a tacky appearance to mixed compounds.