Defects play important roles in semiconductor photocatalysis, which not only can act as active sites but also serve as recombination centers for electrons and holes. The rational control of defects appears to be particularly important. Defect engineering in semiconductor electron-hole separation is complex but vital for photocatalysis, and the exact control of defects still presents a great challenge. This review endeavors to clarify the inherent functionality of different defects such as bulk defects and surface/interface defects. The common defects such as oxygen vacancies and M n + defects have been summarized and discussed. The controllable creation of defects could lead to defect channels via defect-strain coupling, defect-defect and defect-electron interactions, which contribute to the enhancement in electronic conductivity and the separation of photogenerated electron-hole pairs. The deep understanding of defects can consolidate the fundamental photocatalytic theory and provide new insights for rationally designing defect-engineered semiconductor photocatalytic materials with satisfactory performance. This review summarizes the inherent functionality of bulk, surface and interface defects, and their contributions towards mediating electron-hole separation in semiconductor photocatalysis.