This book describes the underlying scientific fundamentals and principal phenomena associated with persistent spectral hole-burning in solids, and presents an overview of possible future applications to optical storage of digital data and optical signal processing. Organization of the material is by the general physical mechanism responsible for the formation of persistent spectral holes. After a description of the basic principles and methods of hole-burning, with examples from photochemical processes in crystalline and amorphous hosts, the unusual proton tunneling phenomena that occur in hydrogen-bonded polymers and glasses are described. Persistent spectral hole-burning in inorganic materials due either to photoionization or to photophysical effects is then summarized, followed by a detailed discussion of nonphotochemical hole-burning mechanisms for electronic transitions in amorphous solids. The book concludes with a description of potential applications to data storage and optical processsing using frequency-domain, holographic, and electric field techniques. Readers of this volume will gain a detailed appreciation of both the generality of the persistent spectral hole-burning phenomenon and the power of the technique in studying microscopic dynamics and mechanisms of phototransformation in low-temperature solids.