The siliceous, potassic igneous rocks that contain more than ~69% SiO2 and typically 3.5--5.5% K2O-the granites and rhyolites-now follow basalts in their importance to earth scientists. Though the 1970's could best be termed a 'Decade of Basalts' <Hart and Allegre, 1980>, an increasing number of earth scientists are now turning their attention to the more siliceous rocks. The present collection of papers on granites and rhyolites includes geological and geophysical field studies, physical and chemical experimental studies, analysis of both rocks and the constituent minerals for major and minor elements and various isotopic ratios, and consideration of pertinent plate tectonic processes and environments. Processes which generate granitic-rhyolitic melts and mechanisms of emplacement of these melts in the upper crust or at the earth's surface are of paramount interest here. This special issue is a sample of modern research and concepts about granites and rhyolites.