The chapter presents an overview of the background information needed to formulate and analyze heat transfer in semitransparent materials systematically and then reviews the literature in some specific problem areas. Primary emphasis is placed on semitransparent solids, although the principles presented are general and apply to any phase. The chapter also discusses the wide variety of applications and involves nature of heat transfer phenomena in semitransparent condensed phases. The radiation characteristics of semitransparent materials are not only dependent on surface but also volume phenomenon since some of the emitted radiation originates at considerable depths. In short, the radiation characteristics depend on the spectral absorption coefficient and index of refraction, thickness, boundary conditions, and temperature distribution. The transient heating of semitransparent materials under idealized conditions in which the emission of radiation from the material is neglected is also discussed. This implies that the body is cold and the rate of emission of radiation per unit volume is negligible compared to absorption. This simplification limits the applicability of analysis and results to those early stages of heating during which temperatures have not raised high enough to render the assumption invalid. The chapter also presents an analysis to show the influence of physical parameters on the temperature field in a semitransparent solid irradiated from a high temperature source such as the sun. The results intend to aid the designer of solar collectors in selecting the optimum material by indicating physical parameters, which determine maximum efficiency of solar energy conversion in different semitransparent solids.