An understanding of the mechanical properties of unsaturated soils is crucial for geotechnical engineers worldwide, as well as to those concerned with the interaction of structures with the ground. This book deals principally with fine-grained clays and silts, or soils containing coarser sand and gravel particles but with a significant percentage of fines.The study of unsaturated soil is a practical subject, linking fundamental science to nature. Soils in general are inherently variable and their behaviour is not easy to analyse or predict, and unsaturated soils raise the complexity to a higher level. Even amongst practicing engineers, there is often lack of awareness of the intricacies of the subject. This book offers a perspective of unsaturated soils based on recent research and demonstrates how this dovetails with the general discipline of soil mechanics.Following an introduction to the basic soil variables, the phases, the phase interactions and the relevance of soil structure, an up-to-date review of laboratory testing techniques is presented. This includes suction measurement and control techniques in triaxial cell testing. This is followed by an introduction to stress state variables, critical state and theoretical models in unsaturated soils.