Oxides have gained increasing interest in surface science during recent years because of their important role in applications. In the first part of the lecture we review the current knowledge on morphology and structure of surfaces of bulk single crystals as well as oxide films. The interaction of oxide surfaces with molecules is thoroughly discussed and the role of defects on adsorption is highlighted. In a further part, structure and morphology of deposited aggregates on clean and modified substrates are discussed. Such systems may serve as models for heterogeneous catalysts. Electronic structure as a function of the size of the deposited particle is studied, as well as size dependent adsorption properties and reactivities.