On the basis of experimental data obtained by testing rigid thermosetting plastics under plane stress conditions the authors analyze the applicability to these plastics of the first and second classical theories of strength as well as the theories of Mohr, Yagn, Balandin, and Mirolyubov. A new strength criterion, a special case of the criterion proposed previously by one of the authors, is recommended as being less complicated, but still accurate enough for practical computations.