  The electronic structure of transition metal oxides featuring correlated
electrons can be rationalized within the Zaanen-Sawatzky-Allen framework.
Following a brief description of the present paradigms of electronic behavior,
we focus on the physics of rare earth nickelates as an archetype of complexity
emerging within the charge transfer regime. The intriguing prospect of
realizing the physics of high $T_c$ cuprates through heterostructuring resulted
in a massive endeavor to epitaxially stabilize these materials in ultra-thin
form. A plethora of new phenomena unfolded in such artificial structures due to
the effect of epitaxial strain, quantum confinement, and interfacial charge
transfer. Here we review the present status of artificial rare-earth nickelates
in an effort to uncover the interconnection between the electronic and magnetic
behavior and the underlying crystal structure. We conclude by discussing future
directions to disentangle the puzzle regarding the origin of the
metal-insulator transition, the role of oxygen holes, and the true nature of
the antiferromagnetic spin configuration in the ultra-thin limit.
