This chapter deals with the control of the flow of energy between the interior and the exterior of the building, through the enclosure, to achieve thermal comfort. Energy is the ability to or capacity for doing work, and heat is a form of energy that appears as molecular motion in substances or as electromagnetic radiation in space. The principal mechanism of heat transfer in multilayer glazing is thermal radiation from a warm pane of glass to a cooler pane. Heat transfer through the enclosure can be by conduction, radiation, or convection. Conduction is the process of heat transfer in a solid or a fluid at rest by a direct molecular interaction between adjacent molecules. Convection is the process of heat transfer by flowing and mixing motions in fluids. Heat transfer by convective effects occurs when there is a temperature difference between the internal and external environments of a building.