This review summarizes the decade-long advancements in defect-engineered g-C₃N₄ for solar catalytic applications, emphasizing the roles of crystallinity and defect traps in achieving precise "customization" of defective g-C₃N₄. It discusses the critical insights into defect traps, exploring defect-induced states and photocarrier transfer kinetics of g-C₃N₄. The review proposes the prospect and outlook for precise defective g-C₃N₄ "customization." Graphitic carbon nitride (g-C₃N₄) has emerged as a universal photocatalyst for sustainable carbo-neutral technologies. However, it faces challenges such as insufficient supply of thermodynamically active photocarriers due to its inferior solar harvesting ability and sluggish charge transfer dynamics. The "all-in-one" defect engineering strategy has been shown to significantly alleviate these issues by simultaneously improving textural uniqueness and intrinsic electronic band structures. Defect engineering involves introducing impurities or regulating atom periodicity in semiconductors, which has proven effective in tailoring the electronic band structures, optical properties, and conductivity of photocatalysts. Various defect types, including vacancies, non-metallic and metallic dopants, functional group grafting, and crystallinity improvement, have been explored to enhance solar harvesting ability, efficient photocarrier transfer, and surface area with more active sites. The review highlights the importance of defect states, particularly shallow defect traps identified by femtosecond transient absorption spectra (fs-TAS), in enhancing photocatalytic performance. It discusses the design principles of defect engineering, including the creation of abundant active sites, enhanced solar harvesting ability, and efficient transport. The review also covers the effects of C and N vacancies on the electronic structure and photocatalytic performance of g-C₃N₄. C vacancies can enhance electronic polarization and improve photocarrier transport, while N vacancies can induce new energy levels and defect states, which can either enhance or hinder photocatalytic performance depending on their depth. The review further explores the synergistic effects of both C and N vacancies, demonstrating that their combined modification can significantly enhance the solar utilization and photocatalytic performance of g-C₃N₄. Non-metal dopants such as C, P, S, O, B, and F have also been shown to optimize the electronic structure, enhance visible-light harvesting, and improve charge separation efficiency. Overall, the review provides a comprehensive overview of the advancements in defect-engineered g-C₃N₄ for solar catalytic applications, emphasizing the importance of precise defect customization for future research and development.This review summarizes the decade-long advancements in defect-engineered g-C₃N₄ for solar catalytic applications, emphasizing the roles of crystallinity and defect traps in achieving precise "customization" of defective g-C₃N₄. It discusses the critical insights into defect traps, exploring defect-induced states and photocarrier transfer kinetics of g-C₃N₄. The review proposes the prospect and outlook for precise defective g-C₃N₄ "customization." Graphitic carbon nitride (g-C₃N₄) has emerged as a universal photocatalyst for sustainable carbo-neutral technologies. However, it faces challenges such as insufficient supply of thermodynamically active photocarriers due to its inferior solar harvesting ability and sluggish charge transfer dynamics. The "all-in-one" defect engineering strategy has been shown to significantly alleviate these issues by simultaneously improving textural uniqueness and intrinsic electronic band structures. Defect engineering involves introducing impurities or regulating atom periodicity in semiconductors, which has proven effective in tailoring the electronic band structures, optical properties, and conductivity of photocatalysts. Various defect types, including vacancies, non-metallic and metallic dopants, functional group grafting, and crystallinity improvement, have been explored to enhance solar harvesting ability, efficient photocarrier transfer, and surface area with more active sites. The review highlights the importance of defect states, particularly shallow defect traps identified by femtosecond transient absorption spectra (fs-TAS), in enhancing photocatalytic performance. It discusses the design principles of defect engineering, including the creation of abundant active sites, enhanced solar harvesting ability, and efficient transport. The review also covers the effects of C and N vacancies on the electronic structure and photocatalytic performance of g-C₃N₄. C vacancies can enhance electronic polarization and improve photocarrier transport, while N vacancies can induce new energy levels and defect states, which can either enhance or hinder photocatalytic performance depending on their depth. The review further explores the synergistic effects of both C and N vacancies, demonstrating that their combined modification can significantly enhance the solar utilization and photocatalytic performance of g-C₃N₄. Non-metal dopants such as C, P, S, O, B, and F have also been shown to optimize the electronic structure, enhance visible-light harvesting, and improve charge separation efficiency. Overall, the review provides a comprehensive overview of the advancements in defect-engineered g-C₃N₄ for solar catalytic applications, emphasizing the importance of precise defect customization for future research and development.