  Electronic phase behavior in correlated-electron systems is a fundamental
problem of condensed matter physics. We argue here that the change in the phase
behavior near the surface and interface, i.e., {\em electronic reconstruction},
is the fundamental issue of the correlated-electron surface or interface
science. Beyond its importance to basic science, understanding of this behavior
is crucial for potential devices exploiting the novel properties of the
correlated systems. % We present a general overview of the field, and then
illustrate the general concepts by theoretical studies of the model
heterostructures comprised of a Mott-insulator and a band-insulator, which show
that spin (and orbital) orderings in thin heterostructures are generically
different from the bulk and that the interface region, about three-unit-cell
wide, is always metallic, demonstrating that {\em electronic reconstruction}
generally occurs. % Predictions for photoemission experiments are made to show
how the electronic properties change as a function of position, and the
magnetic phase diagram is determined as a function of temperature, number of
layers, and interaction strength. Future directions for research are also
discussed.
