In this book, we present concepts and phenomena of metal oxide materials in nanostructured forms, that is ultra-thin films of thickness < 10 nm and nanoparticles of various shapes. Even in bulk form, oxides are characterized by a great variety of stoichiometries, structures, and thus, diverse physical and chemical properties. In oxide nanosystems, additional degrees of freedom are provided by the variable size dimension, the significance of interfaces to substrates and/or the environment, and by the morphology of surfaces and particles; these extra degrees of freedom may be used for modifying properties and for designing desired functionalities. In nanostructured systems, a large proportion of atoms is at the surface; thus, surface properties become particularly important. This is also reflected in the methodology that is employed for the experimental characterization: surface science techniques are widely used for obtaining atomic scale information of nanosystems and they are therefore prominently discussed here in this book. The progress in the fabrication of epitaxial oxide thin films on crystalline substrates by vacuum-based deposition methods during the last two decades has opened up the way to prospective applications in an all-oxide electronics, in all-oxide epitaxial thin film batteries and in the fabrication of epitaxial oxide heterostructures, the latter similar to the well-known heterostructures of semiconductor technology materials. Progress in the wet chemical preparation methods has enabled the reproducible fabrication of oxide nanoparticles of various forms and shapes. Their shape-dependent properties are finding use in chemical applications such as catalysis or biological recognition systems as well as in optoelectronic devices. The discussion of epitaxial oxide ultra-thin films and oxide nanoparticle ensembles thus forms a focus in this book.