Abstract: An unsaturated soil is a state of the soils. All soils can be partially saturated with water. Therefore, constitutive models for soils should ideally represent the soil behaviours over entire ranges of possible pore pressure and stress values and allow arbitrary stress and hydraulic paths within these ranges. The last two decades or so have seen significant advances in modelling behaviours of unsaturated soils. A review of constitutive models for unsaturated soils is presented. In particular, it focuses on the fundamental principles that govern the volume change, shear strength, yield stress, water retention and hydro-mechanical coupling. Alternative forms of these principles are critically examined in terms of their predictive capacities for experimental data, the consistency between these principles and the continuity between saturated and unsaturated states.