This article summarizes the developments of thermoluminescence (TL) as a means of determining the absolute age of certain minerals and rocks of the earth's crust, through the science of geochronology. TL is the property that certain substances emit by visible radiation when heated to a level just below the point where they would begin to incandesce. A brief historical overview is given from its first description in 1663 by Boyle using diamonds through the qualitative experiments early this century to the quantitative applications that came with the invention of photomultiplier tubes in the 1940s. The underlying theory of the TL phenomenon is described in terms of the quantum theory of radiation and the properties of crystalline substances and semi- conductors. Different minerals produce characteristic colors when excited in this process. The equipment required for generating TL and making quantitative measurements is described. The characteristics of these measurements in the form of glow curves are then presented for practical geological application which, up until the time of this article, had generally focussed on stratigraphic correlations and age determinations of calcium deposits.