The first description of the bound rubber phenomenon was by Twiss in 1925, who made the observation that the resistance of carbon black-natural rubber mixes to solvents was related to improved mechanical properties. Boiry studied many of the factors influencing the insolubilization of NR by fillers including type and amount of fillers, and mixing and testing variables. In 1937 J. H. Fielding of Goodyear developed a so-called “bound rubber” test because of his interest in the possibility of chemical bond formation between fillers and rubber. During the start of the U.S. synthetic rubber program, Baker and Walker reported in 1945 an insolubilization of SBR when mixed with carbon black significantly greater than the amount of normal gel in the unfilled elastomer. They were also the first to report that the amount of gel increased with increasing molecular weight and that a selective adsorption of high molecular weight material occurred. Since that time, many investigations have confirmed these findings with other elastomers, and theories of “bound rubber” formation have been based on these observations. The early concept that “bound rubber” is a gel of carbon black particles held together in a three-dimensional lattice by longer interparticle polymer molecules is still valid. The nature of the segmental attachments of the polymer molecules to the filler surfaces now appears to be both physical and chemical, depending upon filler surface activity and chemical functionality, and the chemical composition and functionality of the elastomer. Regardless of the type of interaction, the bonding is essentially permanent and can only be disrupted by extraction with good solvents at high temperatures.