Solid-state systems are frequently classi?ed according to their physical, str- tural or chemical properties. Such schemes are extremely helpful since pr- erties related to any such classi?cation are typically known and facilitate id- tifying solids with special material classes. The best-known examples of these schemes are conductivity or resistivity measurements by means of which m- als are easily distinguishable from insulators. However, frequently clear-cut decisions between material classes are not possible, since anisotropy, chemical composition, binding forces and local effects wash out distinct properties and lead to competition or coexistence. Such unresolved situations are especially typical for transition metal oxides that exhibit a variety of ground-state properties in a fascinating way. Here chemical substitution, doping, pressure or temperature effects easily in?uence the physical properties and may, for instance, induce metal/insulator, antif- romagnet/ferromagnet, insulator/superconductor transitions. This situation is analogous to perovskite ferroelectrics and hydrogen-bonded ferroelectrics, where ferroelectric/antiferroelectric transitions occur with chemical substi- tions of one of the constituent sublattices. In addition, glass-like states (dipolar glasses) are observed and relaxor ferroelectricity with a large potential for - plication frequently occurs.