Defect engineering in photocatalytic materials has recently attracted substantial attention owing to the profound influence of defects on light absorption, charge separation, and interfacial reactions. However, the ambiguity regarding the role of defects in modifying the photocatalytic properties remains a long-standing problem. In this review, defect engineering in photocatalytic materials is comprehensively discussed with a focus on the classification, formation, and characterization of defects and their roles in photocatalytic systems. A systematic study of charge density, rate of charge recombination, and dissociative adsorption in defect-rich photocatalysts is also presented. Defects have the ability to alter the mechanisms of photocatalytic reactions, although the triggering of undesired thermodynamically favored back reactions by excess defects can lead to diminished photocatalytic activity. Furthermore, the critical roles of defects in photocatalytic degradation, H₂ production, and bifunctional systems are reviewed. Finally, the challenges and future potential of defect engineering are considered.