In this contribution, we apply a phase-field model to simulate fracture processes in heterogeneous materials. The local material structure is modelled in a diffuse manner, e.g. by a local reduction of the fracture toughness. In order to obtain a physically meaningful crack propagation, the interaction between the length scales of the diffuse microstructure and the regularised crack are studied. We employ a local correction of the fracture toughness assigned to the interface domain. The method is verified in several numerical benchmark problems and applied to simulate fracture processes in heterogeneous materials.