The principle of critical energy (PCE) is a transdisciplinary principle that may be used in all chapters of sciences for superposition and/or cumulation of different external actions (mechanical, thermal, magnetic, electrical, chemical, etc.) It may be used for lifetime evaluation of engineering structures, as well as for living organisms, by taking into account the influences due to pollution, due to internal deterioration, due to preloading or residual stresses, etc. In this paper, we explain what PCE is, how it may be practically used and the interdependences between the terms comprised in its general relation. PCE was applied for solving problems of superposition of external actions and internal deteriorations in the following chapters of sciences: mechanical engineering, thermomechanical-chemical loadings, multiple pollution, medical field and viscoelasticity. It was compared with synergy theory and with catastrophe theory.