Clean energy conversion technologies can power progress for achieving a sustainable future, while functional materials lie at the core of these technologies. In particular, highly efficient electrocatalysts that are also cost-effective are of utmost concern in the development of several clean energy conversion technologies. These include fuel cells and water electrolyzers. It is well recognized that electrocatalysts are needed to greatly accelerate the kinetics of key electrochemical reactions involved in these energy conversion technologies. Perovskite oxides are being used as key materials that have provided impetus in the development of energy conversion and storage, in view of their flexibility in elemental composition. Over 90% of known elements are able to become incorporated into their metal cation sites, resulting in diversified properties. Several strategies can be used to tailor the intrinsic properties of perovskites, leading to the optimization of their catalytic activity for electrochemical reactions. Cation deficiency has received particular attention as a unique strategy since no other elements or phases are involved, thereby avoiding the unknown effects of foreign elements or phases on performance. In this Account, we present our recent contributions to the study of cation-deficient perovskite oxides in the context of their applications in clean energy conversion. This includes oxygen separation through mixed conducting membranes, which is important for clean combustion, electricity generation through both solid oxide fuel cells using chemical fuels and dye-sensitized solar cells, and hydrogen production from water through both solid oxide electrolysis cells at high temperatures and water electrolyzers at room temperature. The journey begins with a discussion of the defect chemistry and charge compensation mechanisms present in cation-deficient perovskite oxides and then we propose benefits from the cation deficiency strategy that can optimize the materials’ properties in terms of electrical conductivity, phase structure, oxygen vacancy concentration, oxygen ion and cation diffusion properties, and proton transportation. Thus, both the sintering behavior of materials and their catalytic activity is dramatically improved for key reactions such as oxygen reduction, oxygen evolution, hydrogen evolution, and iodine reduction. Accordingly, the remarkable increases in performance of the aforementioned energy conversion systems are achieved through adopting cation-deficient perovskites. At the end of this Account, we conclude with some suggestions on how future research can broaden the applications for cation-deficient perovskites.