Fillers may be minerals, such as chalk, silicates, clays and silica or various forms of carbon black. They are classified in terms of their ability to enhance the physical properties of the rubber. This ability is termed reinforcement and a wide spectrum of fillers is available from non-reinforcing to highly- reinforcing.

The non-reinforcing fillers, eg. ground whiting and some clays, are used as cheap diluents which increase the stiffness of the rubber but usually drastically reduce resistance to fatigue.

The reinforcing fillers include the many varieties of carbon black and silica. Both carbon black, and increasingly silica, are used to enhance the properties of tyres, whilst carbon black is used in engineering products, hose and belting. With the exception of foam rubber most latex products do not contain fillers. Reinforcing fillers are used with both natural and synthetic rubbers and some synthetic rubbers (notably SBR) are virtually unusable without filler reinforcement.

There is a very extensive literature on filler reinforcement.