The active element of an organic field effect transistor (OFET) is a polycrystalline transport layer. The crystallites are interrupted by grain boundaries (GB) that can act as traps or barriers to the charge‐carriers. Their impact on charge transport and hence on the performance of the OFET is still not fully understood. Employing kinetic Monte Carlo studies, the authors set up well‐defined test systems and explore how the parameters of the system, for example, the thickness of the GB, their fractional contribution to the overall film, and the energies of the GB relative to the crystallites, affect the performance of the OFET. It is found that these parameters control the position of the Fermi level, which is crucial in controlling whether the charge transport is confined to GB, or whether it takes place as a superposition between filamentary transport in the boundaries and delocalized transport in the crystallites, or as tunneling‐mediated transport across the crystallites. Guidelines for the morphological optimization of the films for these different transport modes are derived.