  The response of oxide thin films to polar discontinuities at interfaces and
surfaces has generated an enormous activity due to the variety of interesting
effects it gives rise to. A case in point is the discovery of the electron gas
at the interface between LaAlO3 and SrTiO3, which has since been shown to be
quasi-two-dimensional, switchable, magnetic and/or superconducting. Despite
these findings, the origin of the two-dimensional electron gas is highly
debated and several possible mechanisms remain. Here we review the main
proposed mechanisms and attempt to model expected effects in a quantitative way
with the ambition of better constraining what effects can/cannot explain the
observed phenomenology. We do it in the framework of a phenomenological model
for understanding electronic and/or redox screening of the chemical charge in
oxide heterostructures. We also discuss the effect of intermixing, both
conserving and non-conserving the total stoichiometry.
