This paper firstly reviews the studies of the effective stress of unsaturated soils and points out that two stress variables are preferred to be chosen to establish a constitutive model at present. The paper also explains the definition of the matric suction, which involves the effects of the capillary component and the adsorptive component. Secondly the paper discusses the strength of unsaturated soils. The shear strength is usually formulated by the extended Mohr-Coulomb Criterion. The tensile strength also affects the soil behavior due to the capillary effect and thus cannot be ignored. Thirdly, the paper reviews the studies of the constitutive models for unsaturated soils, including the models based on the net stress and the matric suction and that based on the Bishop effective stress and the matric suction. The double pore structure model is also discussed. Finally, the paper addresses the applications of the thermodynamic method and the porous media theory in unsaturated soils. It is shown that these two methods-using the thermodynamic principles to establish the constitutive framework, and using the porous media theory to study the complicated coupling effect of unsaturated soils are subjects very worthy to be developed.