Synthetic nanoscaled metal oxides are mainly produced by pyrogenic decomposition of precursors in the gas phase using a hot-wall or plasma reactor. Due to their low production rate and limited scalability, these processes are of minor technical relevance in manufacturing commercial quantities of nanoparticles. The most common and by far the most important industrial process is flame hydrolysis. In this process, a gaseous mixture of a metal chloride precursor, hydrogen and air is introduced in a closed and continuously operated flame reactor. The general mechanism of formation and growth of particles (e.g. silica) occurring in the flame is dominated by nucleation, coalescence (sintering) and coagulation (collision) of primary particles forming aggregated structures. The term 'aggregate' describes clusters of particles held together by strong chemical bonds. Agglomerates, however, are defined as loose accumulations of particles held together by hydrogen bonds for example. Although, a variety of physico-chemical methods exist to characterise pyrogenic oxides, the most important ones are analysis of the specific surface area by the so-called BET method, determination of the aggregate size by transmission electron microscopy (TEM) and characterisation of the phase composition by means of X-ray analysis.