As far as I know, the systematic study on chalcogenide glasses as semiconductors was initiated around the 1950s by a research group headed by B.T. Kolomiets of the Ioffe Physico-Technical Institute. During the half-century that followed, amorphous semiconductors have acquired their own citizenship as an unchallenged, independent area of science. Their applications are being expanded in industry to include many diverse areas, such as solar cells, thin-film transistors, optical data storage, and so on.

The amorphous semiconductors belong to the class of disordered solids which exist in a thermodynamically non-equilibrium state, and their structures are readily affected, in principle, by external disturbances, such as light, heat and pressure, resulting in quasi-stable states. In particular, it has been found recently that photo-induced metastable structural changes are inherent to most amorphous semiconductors, although the observed phenomena are quite diverse.

I, myself, encountered photostructural changes in the beginning of the 1970s and was fully caught by their charms and immersed in the study of these effects for almost a decade. Subsequently, photo-induced metastability attracted a number of scientists, and has grown to become one of the central themes in amorphous semiconductors. In the latest decade, laser technology has achieved remarkable progress, bringing pico- and femto-second technologies into laboratories, and research in this area is entering a new phase both scientifically and technologically. The frontier is expanding to include most of amorphous materials such as chalcogenides, tetrahedrally bonded semiconductors, oxides and so on.

Enough knowledge has accrued in the field to warrant summing up the diverse achievements into a single book. It is a wonderful sight to see that respectable pioneers and up-andcoming young researchers are contributing to this volume. And happiest of all, Dr. Alex Kolobov, who planned and took charge of the overall construction and editing, came from the Kolomiets Group and has been my colleague and intimate friend for 20 years.