A wide variety of inorganic fillers are produced for the rubber industry. The most important are the clays, precipitated silicas and silicates, and the ground and precipitated calcium carbonates. The silicas and silicates provide the broadest particle size range falling into the carbon black range from FEF (N550) to finer than SAF (N110). The clays and calcium carbonates are in the larger carbon black particle size range from coarser than thermal black (N990) to FEF (N550). if particle size were the only important parameter determining the usefulness of rubber fillers, these products would meet the requirements presently served by carbon black. Their failure to be interchangeable with the carbon blacks is due to their lower modulus and reinforcement performance. These deficiencies are caused by the nature of their surfaces, which are generally more polar and hydrated than carbon black. This makes them more difficult to adhere to and interact with the rubber phase. In order to improve the surface interaction of inorganic fillers with hydrocarbon rubbers, a number of new polymer-reactive, surface-treated products have been introduced. The addition of silane coupling agents during mixing has also been recommended. Silane treated clays and talc, and polymer-grafted clay and calcium carbonate are commercially available. These products are better than their base materials. For some applications, they have been suggested as alternatives to the lower reinforcing grades of carbon black. For the higher reinforcing carbon blacks, only the precipitated silicas with silane additives can be considered as alternatives. However, in tire tread applications, the performance of these combinations has not been clearly defined, and the high cost of the silanes makes their use with silica prohibitive. A more economic method for coupling may result from recent research on functionalized polymers capable of reacting with the surface silanol groups of silica. This survey also includes two finely divided carbonaceous fillers made from coal and petroleum coke. Blends of these materials with more reinforcing carbon blacks and other fillers have been recommended as alternatives to the carbon blacks in the thermal to SRF range. A number of commercial fillers have been suggested as alternatives to the lower reinforcing grades of carbon black for some applications. There are no satisfactory substitute products for the medium to high reinforcing grades of carbon black.