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Almost fifteen years have now elapsed since the first observations of per sistent spectral hole-burning in inhomogeneously broadened absorption lines in solids. The fact that the spectral shape of an inhomogeneously broadened line can be locally modified for long periods of time has led to a large number of investigations of low-temperature photophysics and photochemistry that would not have been possible otherwise. Using hole burning, important information has been obtained about a variety of in teractions, including excited-state dephasing processes, host-guest dynam ics, proton tunnelling, low-frequency excitation in amorphous hosts, relaxation mechanisms for vibrational modes, photochemical mechanisms at liquid helium temperatures, and external field perturbations. At the same time, the possibility that persistent spectral holes might be used to store digital information has led to the study of materials …