This Geotechnical Special Publication contains over 215 papers presented during the Fourth International Conference on Unsaturated Soils held in Carefree, Arizona on April 2-5, 2006. These papers document the experience of researchers and practitioners from around the world, concerning a vast array of unsaturated soil problems. Theoretical and methodological advances in laboratory testing of shear strength and volume change behavior, suction measurement techniques, soil water characteristic behavior, constitutive and numerical modeling, microscale modeling, foundation behavior, heave of slabs and pavements, evapotraspirative covers, geophysical applications, liquefaction and soil dynamics, pavements and slopes, soil-atmospheric interaction, desiccation and shrinkage, in situ testing, seepage, flow of water and gas, and design of waste depositories with emphasis on temperature effects are among the topics dealt with in the framework of unsaturated soil behavior. Materials addressed include natural soils, both expansive and non-expansive, chemically stabilized soil, geosynthetics, geocomposite soils, and bentonite products.