A somewhat neglected area of research in the optical properties of solids is the study of thermoluminescence. The experimental method is to measure the temperature variation of the fluorescence which results from warming a sample in the dark after excitation by some form of radiation at low temperatures. Its usefulness lies in the information it furnishes concerning defect and impurity energy levels in the band gap in semiconductors and insulators. The experimental results when combined with supplementary data (obtained with the same equipment), yield information concerning the energy distribution of trapping levels and luminescent centers in the material. The relatively small investment required for experimental equipment, the very large range of potential materials for study, and the fact that comparatively little is being done in this field all combine to make thermoluminescence a good possibility for solid-state research in the smaller colleges.