Strong interactions between elastomer and filler particles result in the so-called bound rubber (BdR), i.e. the fraction of polymer not extractable from uncured filled rubber compounds by a good solvent of the gum elastomer. BdR is an essential characteristic of uncured compounds and a key element in understanding the flow properties of such materials. After a brief review of works that demonstrate how BdR is involved in the particular morphology of uncured filled rubber compounds, the molecular origin of this phenomenon is explained. A kinetic extraction method to assess BdR is described that yields the absolute value of BdR with a compensation for experimental scatter. Results that further demonstrate how bound rubber is related with the rheological properties of filled rubber materials are reported.