Using transmission electron microscopy, we have compared the domain configurations in epitaxial, ferroelectric PbTiO 3 films grown on different substrates and to different film thicknesses. These configurations change considerably between the different samples. In addition, we have characterized the film/substrate interfaces by high-resolution transmission electron microscopy. It is proposed that the elastic properties of the substrate play a role in determining the domain configuration by influencing the accommodation of tilts of the film lattice.