0_8986 Actinide oxides such as uranium dioxide UO₂ and mixed uranium and plutonium dioxide (U,Pu)O₂ are the most commonly used nuclear fuels in power plants.  These materials exhibit a broad range of stability in composition and temperature in which their thermodynamic and thermal-physical properties vary extensively. This extended stability range is controlled by the formation of different types of defects (point defects and clusters) and the related configurational entropy.In reactor operating conditions, nuclear fuels undergo extreme conditions in temperature and irradiation. Nuclear reactions lead to significant changes in their chemical composition, namely over time, with the formation of numerous fission products. In the most common uranium dioxide fuel pellet, the large thermal gradients produced between the center and the periphery of the fuel pellet induce a radial redistribution of the chemical elements. Oxidation and corrosion of the metallic cladding can also occur. It is thus mandatory to optimize the initial oxygen stoichiometry of the nuclear fuels to get the largest stability and to control the complex change in their microstructure and composition during operation. This requires a deep understanding and the modelling of all the physical-chemical phenomena occurring in these fuels. Different codes are being developed to simulate the manufacturing process and the in-pile behavior in normal and accidental conditions for the reactor safety assessment.An overview of the current state of the art on the disorder analysis in actinide oxides will be presented, with particular emphasis on the role of disorder in stabilizing the various phases present in the nuclear fuel. Different experimental and modelling approaches that are used to study the associated defect chemistry and entropy of these complex actinide oxides will be also described. Finally, the open challenges and the future work will be discussed.