We present a theoretical and computational study of mechanisms that screen the depolarising field in ferroelectric materials, namely domains and two-dimensional electron gases. A more realistic electrostatic description of thin films, which accounts for the presence of a substrate has been derived. The electrostatic description of superlattices of alternating ferroelectric and paraelectric layers was also obtained. In both cases a modified Kittel law was observed for the domains which accounts for the substrates and the superlattice geometries. We review the model of coexistence between screening mechanisms for a ferroelectric thin film in a vacuum. Using the modified electrostatics, this model was extended to describe thin films on substrates and superlattices. It was found that the permittivities of the substrates and paraelectric layers extended the range of thicknesses where coexistence between the screening mechanisms can be sustained. First-principles calculations of monodomain thin films and superlattices were performed using the virtual crystal approximation to introduce a two-dimensional electron gas through interfacial defects. We observed the screening of the depolarising field by the two-dimensional electron gas, sustaining the polarisation within the materials.