The defactant concept is a generalization of the Gibbs Adsorption Isotherm. It extends rigorously the reduction of surface energy of materials by surfactants in Gibbs treatment, like soap molecules in water, to all kinds of discontinuities (defects) of matter including surfaces. Then the formation energy of defects like vacancies, dislocations, grain boundaries and surfaces is reduced, if solute atoms (called defactants) are segregating to the discontinuity. During plastic deformation or crack propagation of metals the afore mentioned defects always play a decisive role and segregation of hydrogen changes their formation energy. As a consequence plasticity and cracking exceeds at higher rates known as hydrogen embrittlement. Although the defactant concept is rather general, hydrogen as a defactant is more dangerous because of its high mobility and omnipresence. Experimental results are presented and interpreted via the defactant concept providing a deeper insight into hydrogen effects in metals.