This chapter provides a comprehensive overview of the structural characteristics, defect chemistry, and synthetic approaches to cerium oxide nanocrystals (CeO2-x). Particular emphasis is placed on the role of oxygen non-stoichiometry and the coexistence of Ce4+ and Ce3+ ions providing high redox activity, oxygen storage capacity, and catalytic performance of nanoceria. The interconnection between vacancy concentration and Ce3+/Ce4+ ratio in nanoceria, and size dependence of Ce3+ content are discussed. The chapter further outlines the principal synthetic routes including sol-gel, hydrothermal synthesis, colloidal precipitation, and cation exchange methods highlighting their influence on morphology, size, and stoichiometry of nanoparticles. Special attention is given to mixed oxides such as CeO2-ZrO2 and CeO2-Y2O3, as well as to doping strategies aimed at tuning the content of oxygen vacancies and Ce3+/Ce4+ ratio in ceria nanocrystals. By integrating structural insights with synthesis strategies, this chapter establishes the fundamental principles necessary for rational design of CeO2-x nanomaterials with tailored properties for advanced technological applications.