What would metal oxides do?: To answer this question, we will have to focus on the nature of the metal–oxygen bond as it can vary between being ionic, highly covalent, or metallic. The distinctive variety of electronic, optical, and magnetic properties (i.e., accessible electronic transitions, high dielectric constants, wide bandgaps, tunable electrical and thermal conductivities) often encountered in metal oxides are clearly due to the unique nature of their outer d-electrons. Given all conceivable metal oxide combinations, we may find that it is possible to yield structures with unique metallic (i.e., ReO3, RuO2, LaNiO3), semiconducting (i.e., ZnO, TiO2), and insulating (i.e., BaTiO3, Al2O3, SiO2) properties. Strongly correlated metal oxides may also undergo metal-to-insulator phase transitions (i.e., V2O3, VO2, La1−xSrxVO3) which allow us to further tune their electronic properties. Strongly correlated metal oxides may also demonstrate charge density wave (i.e., K0.3MoO3), defect ordering (i.e., Ca2Mn2O5, Ca2Fe2O5), ferromagnetism (i.e., Fe3O4, CrO2, La0.5Sr0.5MnO3), and antiferromagnetism (i.e., NiO, LaCrO3) properties. Given their versatile optical and electronic activity, metal oxides have been extensively investigated as key components in optics, electrochemistry, photocatalysis, plasmonics, and photovoltaics. Understanding the structural, morphological, and optoelectronic properties of either naturally occurring or synthetically fabricated metal oxides provides a pathway for the development of novel metal oxides with properties that can be tuned to fulfill the efficiency demands of a particular application. Here, we present a comprehensive review of the synthesis, characterization tools, and applications of metal oxide nanostructures and their thin films. We will also discuss the synthesis, characterization, and applications of metal oxide heterostructures. We have wittingly narrowed the scope of this review to only include the use of metal oxides for energy conversion applications. Finally, we will provide our fair assessment on the outlook, challenges, and opportunities we foresee in metal oxides research and applications.