This book presents an account of the course Advances in Nonradiative Processes in Solids held in Erice, Italy, from June 15 to 29, 1989. This meeting was organized by the International School of Atomic and Molecular Spectroscopy of the Ettore Majorana Centre for Scientific Culture. An area of solid state research that continues to attract the attention of experimental and theoretical physicists is that of nradiative relaxation processes of excited solids. The interest in these processes stems from their techlogical relevance, and from the difficulty in the quantitative characterization and differentiation of their various pathways. The decay channels leading to the ground state include the conversion of electronic excitation energy into phon energy, nradiative transfer of excitation energy, upconversion processes, etc. Considerable advances have been achieved in understanding and modeling the radiative process that follow the electronic excitations of solids; the progress in this field has been instrumental in the development of new solid-state devices and laser materials. On the other hand, these advances have underscored the inadequacy in the understanding of the nradiative relaxation processes. This course dealt with the advances in physical modeling, mathematical formalisms and experimental techniques relevant to the quantitative characterization of the various pathways of nradiative relaxation of solids in excited electronic states.