This is the first book from MRS to explore the microstructure of irradiated materials. The book focuses on defect production and accompanying microstructural changes that occur in solids during irradiation with energetic particles (e.g. ions, electrons, neutrons, or gammas). Such irradiation can produce dramatic changes in the properties of materials - changes which in some cases are considered beneficial, and deleterious in others. In either case, the key to understanding the irradiation-driven property changes is the underlying alloy microstructure. Experimental, computational and theoretical perspectives are offered on topics that include: microstructure of irradiated metals; microstructure of irradiated ceramics; and microstructure of irradiated semiconductors. A remarkable number of similarities in the irradiation-produced microstructure of these materials are revealed.