The development of highly efficient and robust electrocatalysts plays a crucial role in the success of sustainable energy conversion technologies, like electrochemical water splitting for hydrogen production. Defect engineering has been considered as an effective strategy to tune the electrocatalysts and enhance the activity. This review covers recent advances in engineering, probing, and understanding the defects in electrocatalysts toward the hydrogen evolution reaction and oxygen evolution reaction. In particular, the role of defects in electrocatalysts have been comprehensively discussed and summarized on the basis of both experimental results and theoretical calculations. We further provide a summary of the computational insight into the role of defects. Finally, we propose the critical challenges and corresponding future directions.