This chapter provides an overview of the important aspects of unsaturated soil behaviour that are of relevance for tropical residual soils. Many tropical soils exist in an unsaturated state due to soil water deficits induced by the tropical climate. The concepts of suction are introduced and how suction can be measured. The role of suction in determining shear strength is described, together with test results, showing the variations of φa and φb for two tropical soils. The volume change behaviour of unsaturated soils (shrinkage, swelling and collapse) is also discussed as this can be of relevance to tropical clay soils.