The "Theory of Strength" as the third volume of "Introduction to Technical Mechanics" is now available in its 3rd edition. The author's chosen approach emphasizes the common foundations of statics, strength theory, and kinetics, thereby facilitating the understanding of the entire field of Technical Mechanics. The textbook addresses, alongside the general prerequisites of strength theory, the fundamental types of stress that are important for immediate engineering practice: tension/compression, torsion, bending, and shear force. These topics are supplemented by simple strength hypotheses, energy considerations, and stability analyses. Illustrative rotationally symmetric calculation models lead to the formulation of the linear-elastic field problem, which forms the basis for the continuum mechanics-based computer programs in strength theory. The book covers the material of the foundational courses.