Decreasing the particle size of a solid to the nanoscale has consequences for its crystalline and electronic structures. It also involves a large extension of the surface area, thus modifying its reactivity. This chapter focuses on the effect of particle size on thermodynamic, structural and electronic characteristics and on redox properties. It is commonly accepted that a decrease in particle size increases the reactivity of materials. This is discussed here through the description of three main iron oxide structural groups, namely spinel (magnetite, maghemite), hematite and goethite, which exhibit specific behaviour vs. size effect. The role of structural defects and nanoparticle morphology in terms of reactivity is highlighted.