Characterization of the relationships between structure and properties of materials is based on the fundamental principle that the structure of the material be determined first, followed by assessments of which structural properties may govern their properties as a function of composition, pressure, temperature and other variables. Whereas this methodology has been successfully applied to further our understanding of crystalline materials, studies of silicate melt structure are often conducted on a somewhat different basis. Rather than from direct structural determination, structure models have been developed from assumed relationships between a specific melt property and its structure. As a result, a multitude of models has evolved - many of which are mutually exclusive. The overall scope of this book is to address properties and processes of magmatic systems from the vantage point of melt structure. To this end available data in chemically increasingly complex systems are reviewed and discussed with the ultimate goal being integration of the simple system data into a model that describes complex systems such as natural magmatic liquids. Thus the book evolves from the simplest possible system, SiO 2 , to complex systems such as natural magmatic liquids. From a petrologic point of view, sufficient data have been obtained so that a general framework of the structure of magmatic liquids is in place. This framework is based on the same principles as those of crystal chemistry, modulated by the absence of long range order in amorphous material, and systematic relationships between structure and properties can be discerned at least at atmospheric pressure.