Abstract: Stress variables serve as the foundation for describing the mechanical properties of unsaturated soils, including deformation, strength, and water retention capacity. Establishing appropriate stress variables constitutes a fundamental subject in unsaturated soil mechanics and has received long-term attention from numerous scholars. To provide valuable references for future research, this paper systematically sorts and reviews effective stress and stress state variables in the literature, focusing on stress variable construction methods. It also detailedly introduces new developments in validating stress variables and effective stress parameters in detail. The research findings indicate: (1) Five primary methods exist for constructing stress variables describing unsaturated soil mechanical characteristics: phenomenological method, mechanical effect equivalence method, equilibrium equation method, power conjugate method, and thermodynamic method. Each method yields multiple sets or multiple individual stress variables for selection and application. (2) Selecting stress variables requires consideration of four aspects: theoretical soundness, practical simplicity, logical consistency, and thorough validation—collectively termed the four principles for stress variable selection. No single aspect should dominate the choice. These principles are applied to evaluate the strengths and weaknesses of multiple stress variable sets and effective stress parameters documented in the literature. (3) Guided by the principle that practice serves as the ultimate criterion for testing the validity and quality of theories, this study provides a detailed introduction to new developments and insights in verifying stress variables (including stress state variables and effective stress parameters). It analyzes verification data and results from hundreds of triaxial shear tests and direct shear tests on unsaturated soils. It concludes that the saturation at failure, the effective saturation at failure, and the formula (5) and (6) as effective stress parameters is inappropriate. Through both necessary and sufficient condition tests, it is concluded that net total stress and suction are two independent stress variables that can conveniently describe the mechanical properties of unsaturated soils. Effective stress parameters correlate with soil type, suction, net confining pressure (or net vertical stress), and stress path. For many soils, effective stress parameters measured in shear strength tests under low suction conditions exhibit values significantly greater than 1 or extremely small, lacking clear discernible patterns. Currently, the current feasible approach involves determining effective stress parameters using unsaturated soil triaxial drained shear tests or direct shear tests that control suction to account for the aforementioned influencing factors.