Abstract: The unsaturated soil strength is the most important issue in the unsaturated soil theory and its engineering applications. But advances in the true triaxial tests of the unsaturated soils have been very slow due to the control-measurement technique and the balance time of the matric suction under complex stress states. This paper briefly reviews the rigid and flexible and compound true triaxial apparatuses and the existing true triaxial tests for the unsaturated soils. It is shown that both Mohr-Coulomb and circumscribed Drucker-Prager strength criteria are not valid for the true triaxial tests of the unsaturated soils. Different types of true triaxial tests should be conducted, particularly, for non-saturated clays. In order to improve the theoretical basis for the study of the unsaturated soils, a true triaxial strength criterion of the unsaturated soils in accordance with the actual stress conditions in the field should be established, based on the stress state variables and the strength characteristics of the unsaturated soils.