This review summarizes the decade-milestone advancements in defect-engineered graphitic carbon nitride (g-C₃N₄) and emphasizes the roles of crystallinity and defect traps in achieving more precise "customization" of g-C₃N₄ for solar catalytic applications. The paper delves into the defect-induced states and photocarrier transfer kinetics of g-C₃N₄, highlighting the prospect and outlook for precise defective g-C₃N₄ customization. Key topics include: 1. **Defect Engineering**: The introduction of impurities or regulation of atom periodicity to tailor the electronic band structures, optical properties, and conductivity of g-C₃N₄. 2. **Vacancies**: C and N vacancies are discussed in detail, including their formation mechanisms, effects on electronic band structures, and impact on photocatalytic activities. 3. **Non-Metal Dopants**: C, P, S, O, B, and F dopants are explored for their roles in optimizing electronic band structures and enhancing photocatalytic performance. 4. **Crystallinity and Defect Traps**: The importance of crystallinity and the identification of shallow defect traps through femtosecond transient absorption spectra (fs-TAS) are emphasized. 5. **Design Principles**: The principles for designing defective g-C₃N₄, including abundant active sites, enhanced solar harvesting, and efficient photocarrier transport, are outlined. 6. **Theroretical Guidance**: The use of density functional theory (DFT) calculations to understand defect roles and redox mechanisms is highlighted. 7. **Challenges and Solutions**: The challenges in solar light absorption, photocarrier transfer kinetics, and recombination in bulk and surface phases are addressed, with defect engineering as a comprehensive solution. The review aims to provide a comprehensive understanding of defect engineering in g-C₃N₄, offering insights for future research and applications in solar catalysis.This review summarizes the decade-milestone advancements in defect-engineered graphitic carbon nitride (g-C₃N₄) and emphasizes the roles of crystallinity and defect traps in achieving more precise "customization" of g-C₃N₄ for solar catalytic applications. The paper delves into the defect-induced states and photocarrier transfer kinetics of g-C₃N₄, highlighting the prospect and outlook for precise defective g-C₃N₄ customization. Key topics include: 1. **Defect Engineering**: The introduction of impurities or regulation of atom periodicity to tailor the electronic band structures, optical properties, and conductivity of g-C₃N₄. 2. **Vacancies**: C and N vacancies are discussed in detail, including their formation mechanisms, effects on electronic band structures, and impact on photocatalytic activities. 3. **Non-Metal Dopants**: C, P, S, O, B, and F dopants are explored for their roles in optimizing electronic band structures and enhancing photocatalytic performance. 4. **Crystallinity and Defect Traps**: The importance of crystallinity and the identification of shallow defect traps through femtosecond transient absorption spectra (fs-TAS) are emphasized. 5. **Design Principles**: The principles for designing defective g-C₃N₄, including abundant active sites, enhanced solar harvesting, and efficient photocarrier transport, are outlined. 6. **Theroretical Guidance**: The use of density functional theory (DFT) calculations to understand defect roles and redox mechanisms is highlighted. 7. **Challenges and Solutions**: The challenges in solar light absorption, photocarrier transfer kinetics, and recombination in bulk and surface phases are addressed, with defect engineering as a comprehensive solution. The review aims to provide a comprehensive understanding of defect engineering in g-C₃N₄, offering insights for future research and applications in solar catalysis.