Transition metal (TM) compounds present a unique class of solids. The physics of these materials is extremely rich. There are among them good metals and strong, large-gap insulators, and also systems with metal–insulator transitions. Their magnetic properties are also very diverse; actually, most strong magnets are transition metal (or rare earth) compounds. They display a lot of interesting phenomena, such as multiferroicity or colossal magnetoresistance. Last but not least, high-Tc superconductors also belong to this class.

Transition metal compounds are manifestly the main area of interest and the basis for a large field of physical phenomena: the physics of systems with strong electron correlations. Many novel ideas, such as Mott insulators, were first suggested and developed in application to transition metal compounds.

From a practical point of view, the magnetic properties of these materials have been considered and used for a long time, but more recently their electronic behavior came to the forefront. The ideas of spintronics, magnetoelectricity and multiferroicity, and high-Tc superconductivity form a very rich and fruitful field of research, promising (and already having) important applications.

There are many aspects of the physics of transition metal compounds. Some of these are of a fundamental nature – the very description of their electronic structure is different from the standard approach based on the conventional band theory and applicable to standard metals such as Na or Al, or insulators or semiconductors such as Ge or Si.