Abstract: It is the effective stress that controls the deformation and strength of geomaterials. The principle of effective stress has been utilized extensively in geotechnical engineering. However, theoretical explanation for the effective stress controlling the shear strength of geomaterials is so far by no means sufficient and is doubted. In order to improve this situation, a comprehensive literature review is carried out. This results in a finding of a positive correlation between macroscopic shear strength and interparticle friction strength. The adhesion theory of friction in tribology is adopted to derive the formulae of the junction growth of an interparticle contact and the shear strength under both drained and undrained conditions. Through a comparative study of the influences of the stress states under both conditions on the shear strength, an effective stress expression in consistent with Terzaghi’s expression is achieved. On this basis, the concept and the definition for both the effective stress and the principle of effective stress are discussed.