The fundamental connexion between thermoluminescence, phosphorescence and electron traps in solids has been investigated. Thermoluminescence and long-period phosphorescence arise from the release of electrons from metastable levels or traps. By studying the thermal stability of trapped electrons and the probability of release from traps of different depths, methods have been developed for finding the depths of electron traps in phosphors. The main experiment consists in exciting the phosphor at low temperatures until all the traps are filled; the phosphor is then warmed at a steady rate and the light emitted while warming is measured as a function of the temperature. The results show that the trap distribution in impurity phosphors such as willemite and the alkaline earth sulphides are, in general, complex, and extend over a range often as wide as 0\cdot 2-1\cdot 0 eV. The probability of release of an electron from a trap of depth E at temperature T is of the form se-E/kT, where s is a constant. Values of s for alkaline earth and zinc sulphides are in the neighbourhood of 108± 1 sec.-1.