This chapter discusses the electronic structure and luminescent properties of stoichiometric and non-stoichiometric cerium oxide nanocrystals (CeO2-x). Using stationary and time-resolved luminescence spectroscopy, the effects of oxygen vacancies and Ce3+ formation on the optical properties of ceria nanocrystals are shown. Ce4+ → Ce3+ reduction induced by oxygen deficiency leads to the formation of Ce3+-V0-Ce3+ complexes, which are responsible for the characteristic Ce3+ 5d–4f luminescence in ceria nanocrystals. The content of Ce3+ ions increases as the size of nanoparticles decreases, leading to size dependence in Ce3+ luminescence intensity. Doping ceria nanocrystals with Y3+ or Zr4+ ions enhances oxygen vacancy concentration, further increasing the number of Ce3+ optical centers. A generalized energy level scheme is proposed for CeO2-x nanoparticles highlighting the interplay between 4f and 5d states of Ce3+ ions and defect-related levels. The results provide critical insights into the mechanisms governing luminescence in CeO2-x nanocrystals and the role of defect engineering in tuning their luminescent properties.