It is widely recognized that the presence of defects in crystals and other solid materials can have a profound effect upon their chemical and physical properties and, consequently, that defects have a major impact on the practical utility of many technological materials. The presence of defects in a crystalline material implies the presence of disorder, and the extent of such disorder can range from very minor, such as the occurrence of Schottky defects in a crystal of sodium chloride, to maximum disorder, as in an amorphous material. The focus of this overview is on systems that are partially disordered, spanning the range between—but not including—sodium chloride and an amorphous material. Even within this range, the aim is not to be comprehensive, since for space reasons we have restricted our coverage to inorganic materials and hybrid inorganic-organic systems. The choice of this topic stems from both its fundamental and practical importance; it is also a very timely topic. For example, there is a great deal of current interest in complex, partially ordered materials such as the surfactant-mediated mesoporous silicas, biominerals, and hybrid organic-inorganic composites. Research on such materials has presented challenges that cannot easily be addressed by characterization tools that have been developed for well-ordered materials. The same situation is found in other areas such as carbons (including nanotubes) and glassy metal oxides.