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:Water retention and flow characteristicsShear strength and stiffness at various temperaturesState-dependent elasto-plastic constitutive modellingField monitoring and engineering applicationsThis 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 activitiessmall strain stiffness considering the influence of wetting- drying cycles and recent suction history, such as that due to climate changesuction effects on dilatancy and peak shear strengthcyclic thermal effects on soil behaviourstate- dependent elastoplastic constitutive modelling of monotonic and cyclic behaviourengineering 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