The subject of this paper is the theory of strength, which tries to answer the question of when two elements of the same material, regardless of the stress state, will be in the same state, ie have the same mechanical properties. We use strength theories to more easily describe the complex state of stress an element is in, this means that, for example, we want to reduce the triaxial stress state to uniaxial through different relationships. Efforts are made to cover materials that differ in their behavior ie fragile stone on one side and ductile steel on the other. The introductory section provides a few examples that led us to think about simplifying this otherwise significant and complex problem. Further, the first five theories of strength, most commonly used in technical practice and Mohr's theory of strength are explained in the paper, the advantages and disadvantages of these theories and the situations in which the particular theory is most commonly used are listed. Excerpts are given which lead to the formulation of equivalent stress according to each theory. Also, in the third chapter, two numerical examples are solved in which five strength theories are covered. In these examples, we sought critical stress on variously loaded supports, one of which is a hollow tube cross section and the other a full T- shaped cross section.