Unsaturated soil is a three-phase material that is ubiquitous on the Earth’s surface and exhibits complex behaviour, which becomes more complex in response to the Earth’s changing climate and increasing engineering activities. This is because the former affects its moisture and temperature conditions significantly and the latter governs its stress state and suction condition. This book is designed to meet the increasing challenges of climate change and engineering activities by covering the mechanics and engineering of unsaturated soil in a logical manner. It comprises four major parts: (1) water retention and flow characteristics; (2) shear strength and stiffness at various temperatures; (3) state-dependent elasto-plastic constitutive modelling; and (4) field monitoring and engineering applications. This second edition uniquely covers fundamental topics on unsaturated soil that are not covered in other similar books, including: • the state-dependency of soil-water retention behaviour and water permeability functions, such as dependence on engineering activities; • small strain stiffness considering the influence of wetting-drying cycles and recent suction history, such as that due to climate change; • suction effects on dilatancy and peak shear strength; • cyclic thermal effects on soil behaviour; • state-dependent elastoplastic constitutive modelling of monotonic and cyclic behaviour; and • engineering applications such as the South-to-North Water Transfer Project; an earthen landfill cover system devoid of geomembrane in the Xiaping landfill, Shenzhen; and a 15-m-deep multi-propped excavation in Tianjin, China. Charles W.W. Ng is Chair Professor and the CLP Holdings Professor of Sustainability at The Hong Kong University of Science and Technology. He is also a Fellow of the Royal Academy of Engineering, an Editor-in-Chief of the Canadian Geotechnical Journal and the immediate past-President of the International Society for Soil Mechanics and Engineering (2017–2022). Chao Zhou is Associate Professor at The Hong Kong Polytechnic University. Junjun Ni is Professor at Southeast University in Nanjing, China.