In crystals as diverse as sodium chloride, silicon dioxide, sold xenon, pyrene, arsenic triselenide, and silverchloride, the fundamental electronicexcitation (exciton) islocalized within its own lattice distortion field veryshortly after its creation. This book discusses thestructure if the self-trapped exciton (STE) and itsevolution along the path of its return to the ground stateor to a defect state of crytal. A comprehensive review ofexperiments on STEs in a wide range of materials has beenassembled, including extensive tables of data. Throughout, emphasisis given to the basic physics underlying variousmanifestations of self-trapping. The role of the spontaneoussymmetry-breaking or "off-center"relaxation in STEstructure is examined thoroughly, and leads naturally to thesubject of lattice defect formation as a product of STErelaxation. The theory of STEs is developed from alocalized, atomistic perspective using self-consistentmethods adapted from the theory of defects in solids. Atthis time of rapid progress in STEs, researchers willwelcome the first monograph dedicaded solely to this topic.