Components for passive solar energy utilization can be generally treated as consisting of translucent insulation, radiation control, absorber and storage. Although even in central Europe the total radiation upon a building amounts to more than the heating energy needed during the winter season, closer inspection shows that the available storage capacity is not sufficient to cover the entire load by radiation: auxiliary heat is always required. On the other hand, overheating must be avoided by means of control devices. Components in use at present are: windows, conservatories and, to a lesser extent, trombe walls. Newer developments are based on materials exhibiting high light transmission and good heat insulation properties. Such materials can be used as outside insulation of old building stock and offer prospects for significant energy savings.