The　electronic　structure　and　photoactivation　process　in　N-doped　TiO　2　is　investigated.　Diffuse　reflectance　spectroscopy　(DRS),　photoluminescence　(PL),　and　electron　paramagnetic　resonance　(EPR)　are　employed　to　monitor　the　change　of　optical　absorption　ability　and　the　formation　of　N　species　and　defects　in　the　heat-　and　photoinduced　N-doped　TiO2　catalyst.　Under　thermal　treatment　below　573　K　in　vacuum,　no　nitrogen　dopant　is　removed　from　the　doped　samples　but　oxygen　vacancies　and　Ti3+　states　are　formed　to　enhance　the　optical　absorption　in　the　visible-light　region,　especially　at　wavelengths　above　500　nm　with　increasing　temperature.　In　the　photoactivation　processes　of　N-doped　TiO2,　the　DRS　absorption　and　PL　emission　in　the　visible　spectral　region　of　450-700　nm　increase　with　prolonged　irradiation　time.　The　EPR　results　reveal　that　paramagnetic　nitrogen　species　(Ns.),　oxygen　vacancies　with　one　electron　(V　o.),　and　Ti3+　ions　are　produced　with　light　irradiation　and　the　intensity　of　Ns.　species　is　dependent　on　the　excitation　light　wavelength　and　power.　The　combined　characterization　results　confirm　that　the　energy　level　of　doped　N　species　is　localized　above　the　valence　band　of　TiO2　corresponding　to　the　main　absorption　band　at　410　nm　of　N-doped　TiO2,　but　oxygen　vacancies　and　Ti3+　states　as　defects　contribute　to　the　visible-light　absorption　above　500　nm　in　the　overall　absorption　of　the　doped　samples.　Thus,　a　detailed　picture　of　the　electronic　structure　of　N-doped　TiO2　is　proposed　and　discussed.　On　the　other　hand,　the　transfer　of　charge　carriers　between　nitrogen　species　and　defects　is　reversible　on　the　catalyst　surface.　The　presence　of　oxygen-vacancy-related　defects　leads　to　quenching　of　paramagnetic　N　s.　species　but　they　stabilize　the　active　nitrogen　species　Ns-.　Seeing　the　light:　Nitrogen　species　in　N-doped　TiO2　act　as　photoactive　centers　for　photogenerated　electron　transfer　to　adjacent　defect　sites.　Their　energy　level　is　localized　above　the　valence　band　of　TiO2　corresponding　to　the　main　absorption　band　at　410　nm　of　N-doped　TiO2　(see　picture;　Ns.=paramagnetic　nitrogen　species,　Vo.=oxygen　vacancy　with　one　electron).　©　2012　WILEY-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim.