Using transmission electron microscopy, we have compared the domain configurations in epitaxial, ferroelectric PbTiO3 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.