The feldspars form the most abundant group of minerals in the crust of the Earth and Moon and also occur in many meteo­ rites. They playa fundamental role in all rock-forming processes at shallow depths, but are rare or absent from the upper mantle. Their detailed study is thus essential for the understan­ ding of such varied processes as magma genesis and differentia­ tion, metamorphism, al teration, erosion and sedimentation. This interest is show by the fact that two previous NATO Advanced Study Institutes on feldspars were held in Oslo in 1962 and in Manchester in 1972. The feldspars are particularly sui table for detailed studies, as they have very simple chemistry and develop some of the most complex microstructures known. The microstructures are often slow to form but are easily preserved, so that they are potentially extremely informative about the geological history of the rocks in which they occur. Furthermore, their study involves physical and chemical methods of increasing sophistication so that the results obtained are not always immediately understandable to research workers outside the field of modern mineralogy. Progress in knowledge about feldspars is probably slower in penetrating the fields of petrology and geochemistry than that on other mineral groups. For these reasons among others, i~ was particularly appropriate to hold a third NATO ASI on feldspars approximately ten years after the last one.