The exploitation of highly efficient, low-cost, and stable electrocatalysts is a key issue of the broad application of green electrocatalytic reactions and efficient energy devices. Recently, modulating the surface structure of electrocatalysts to improve the catalytic activity has attracted a lot of attention. In particular, defect engineering is an important strategy to modulate the surface electronic structure of electrocatalysts. In this review, an overview of defects in metal, carbon materials, transition metal compounds, and defect-decorated catalysts is presented. The defect species, synthesis methods, characterization, and essential defect catalytic mechanism are introduced. Notably, tuning electronic structure to modulate the intermediates’ adsorption energy is highlighted throughout the review. Finally, the design principles for defect electrocatalysts are proposed. The in-depth understanding of the structure–reactivity relationship will provide more profound guidance for the design of defect electrocatalysts and potential application in energy conversion and green synthesis.