Abstract: Strength theory deals with the failure of materials under complex stress states. Summarizing the classical strength theories and two kinds of modern strength theories of concrete and isotropic rock. Isotropic strength theories include shear stress strength theory, octahedral strength theory, and principal stress strength theory. Various major strength theories were discussed, compared, and evaluated through collecting relevant domestic and foreign triaxial experimental data. Based on previous researches, the principal stress-space strength theories of concrete and isotropic rock were prospected.