The development of advanced catalysts for efficient electrochemical energy conversion technologies to alleviate the reliance on fossil fuels has attracted considerable interest in the last decades. Insight into the roles of reactive sites in nanomaterials is significant for understanding and implementing the design principles of nanocatalysts. Recently, the essential role of defects, including vacancies, reconstructed defects, and doped non-metal (or metal)-defect-based motifs, have been widely demonstrated to promote the diverse electrochemical processes (e.g., O2 or CO2 reduction reactions and H2 or O2 evolution reactions). Nevertheless, the in-depth exploration of the underlying defect electrocatalytic mechanism is still in its infancy. This review summarizes the state-of-the-art defect engineering strategies for designing highly efficient electrochemical nanocatalysts with special emphasis on the correlation between defect structures and electrocatalytic properties. Finally, some perspectives on the challenges and future research directions in this promising area are presented. Display omitted Electrocatalytic energy conversion technologies have been widely considered a clean and sustainable way to alleviate the reliance on fossil fuels. The development of efficient and affordable electrocatalysts plays a key aspect in energy conversion processes by lowering the reaction kinetic barriers and thus boosting the efficiency and selectivity of diverse electrochemical reactions (e.g., oxygen and hydrogen evolution reactions and oxygen and carbon dioxide reduction reactions). Recently, defect engineering has emerged as a new strategy for tailoring the electronic structures and interface coordination; however, the role of “defect”-related sites in as-designed electrocatalysts has not yet been fully understood. In this review, we will shed light on the recent advances in tailoring nanomaterials from the aspects of constructing defect-based motifs as active sites for versatile electrochemical energy conversions as well as their underlying mechanism on structure-property correlations. Recently, the essential role of defects, including vacancies, reconstructed defects, and doped non-metal/metal-defect-based motifs, has been widely demonstrated to promote the diverse electrochemical processes (e.g., O2/CO2 reduction reactions and H2/O2 evolution reactions). This review summarizes the state-of-the-art defect engineering strategies for designing highly efficient electrochemical nanocatalysts with special emphasis on the correlation between defect structures and electrocatalytic properties. Finally, some perspectives on the challenges and future research directions in this promising area are presented.