The relative energies and occupancies of valence orbital states can dramatically influence collective electronic and magnetic phenomena in correlated transition metal oxide systems. We review the current state of research on the modification and control of these orbital properties in rare-earth nickelates, especially LaNiO3, a model degenerate d orbital system where significant recent progress has been made. Theoretical and experimental results on thin films and heterostructures are described, including the influence of electronic correlation effects. We highlight the latest approaches to achieving non-degenerate bands and discuss the outlook and applicability of this body of knowledge to other correlated metal oxide systems.
