Washington, D.C., when Congress established a unified geological survey under the Department of the Interior, combining the duties of and replacing four earlier government surveys. The Survey's program included "classification of the public land, and examination of the geological structure, mineral resources, and products of the national domain." It seems entirely appropriate, therefore, that a professional paper summarizing the progress of investigations and synthesizing our present state of knowledge in the important field of eolian deposition be one of those issued to commemorate the Geological Survey's 100th anniversary.Early descriptions and considerations of the characteristics of modern eolian sand deposits, mostly in the great sand seas of the world, date back to the 1880's and 1890's, when pioneer geologists and explorers, including Walther, von Zittel, and Sven Hedin, wrote their classic papers on desert dunes. One or two decades later, interest in eolian processes greatly increased when wind-formed deposits were recognized in ancient sandstones in many parts of the world and in rocks of many ages.Throughout the 20th century, as the science of geology has expanded and the programs of the U.S. Geological Survey have proliferated to keep pace, dune studies have had a similar growth. Work was initially concentrated mostly on the description of dune forms or morphology and on analysis of textural features; by midcentury, however, major contributions had been made to the physics of eolian sand, as exemplified by the classic work of Bagnold, by detailed studies and interpretations of minor eolian structures, by statistical analyses of cross-strata dip directions, and by the development of systems for dune classification. Most recently, interest has been renewed in detailed grain studies, in the study of crossstrata, and in interpretation of dune patterns by means of aerial photographs and Landsat imagery.A major feature of this report on global sand seas is the compilation and comparison of available data based on many different methods of investigation. Evidence at one extreme is obtained from the detailed studies of minute particles and from analysis of individual grains. At the opposite extreme is evidence obtained from remote sensing, in which dune patterns, recorded from approximately 500 miles in space, are compared from one sand sea to another. Furthermore, comparisons are made between ancient and modern deposits with respect to textures, structures, and other characteristics. Criteria for recognizing the eolian origin of various ancient deposits are discussed. The application of these studies to economic problems, which is described in one chapter, clearly illustrates the importance of eolian deposits to our present culture and to human welfare.