In this dissertation, we used a combination of several synchrotron-based x-ray spectroscopic techniques to investigate the effects of strain, ionic defect formation, and heteroengineering in strongly-correlated electronic systems. First, we introduce a method to control and stabilize oxygen vacancies in complex transition-metal oxide thin films. In our approach, we utilized atomic layer-by-layer pulsed laser deposition (ALL laser PLD) from two separate targets to synthesize high-quality single crystalline CaMnO3 films under coherent tensile strain, varying systematically from +0.8% to +4%. An increase of the oxygen vacancy content in the single-crystalline CaMnO3 thin films with applied in-plane strain was experimentally observed using high-resolution soft x-ray absorption spectroscopy (XAS) in conjunction with bulk-sensitive hard x-ray photoelectron spectroscopy (HAXPES). Our experimental results were verified using first-principles theory and atomic core-hole multiplet calculations. Furthermore, our results highlight the importance of protecting the surfaces of CaMnO3 thin-films with thin Pt layers in-situ in order to stabilize the oxygen vacancy content. Next, we discuss the role of oxygen vacancies in driving the metal-insulator transition in LaNiO3 thin films. Here, we also use atomic layer-by-layer pulsed laser deposition (ALL laser PLD) from two separate targets to synthesize high-quality single-crystalline LaNiO3 films with systematically varying thicknesses, ranging from 1 u.c. to 50 u.c. An increase in the oxygen vacancy content was observed with the decreasing LaNiO3 film thickness using XAS. A higher concentration of oxygen vacancies was observed for the ultrathin insulating films (<1.5 u.c.). The experimental results were compared to first-principles theoretical calculations. We found that LaNiO3 exhibits room-temperature metallic behavior for thicknesses down to 1.5 u.c., which is the lowest value reported to date. Finally, we have investigated an atomically-abrupt interface between the paramagnetic LaNiO3 and the antiferromagnetic CaMnO3 thin films. The interface between these two complex oxides exhibits interfacial ferromagnetism, which can be tuned via a thickness-dependent metal-insulator transition in LaNiO3. Here, we used depth-resolved standing-wave photoemission spectroscopy (SW-XPS), scanning transmission electron microscopy (STEM), and XAS to observe a depth-dependent charge reconstruction occurring at the LaNiO3/CaMnO3 interface. Our elemental standing-wave rocking-curve analysis revealed the depth-dependent changes of the Mn and Ni valence states at the interface, yielding increased amounts of Mn3+ and Ni2+ cations at the interface. These results suggest Mn4+-Mn3+ ferromagnetic double exchange and Ni2+-Mn4+ superexchange as possible underlying causes of the emergent interfacial ferromagnetism. / Physics