The charge carrier mobility in the transport channel of an organic transistor is estimated within the framework of a trap-and-release model. The model accounts for the observed dependence of the mobility on the dielectric constant "epsilon" of the gate insulator. This dependence is attributed to both the effective mass of the carrier and the energetic depth of transport traps due to interface defects being functions of "epsilon". These results are used to describe the critical role of the interface between the organic semiconductor and the dielectric material in governing charge transport in organic transistors.