Exergy analysis of systems is extremely rich as it uses the so-called exergy accountancy which takes into account first and second principles of thermodynamics. This exergetic accountancy enables to quantitatively evaluate what is calles qualitatively the energy degradation, i.e. to calculate precisely the consequences of different phenomena of thermodynamical irreversibility, indeed to calculate correctly the thermodynamic losses in a system. Therefore the exergetic accountancy enables to define correctly a thermodynamical yield, expressing the perfection degree, i.e. the thermodynamical quality of any system. Exergy analysis is a way to estblish a true diagnosis concerning the thermodynamical health of a system of any size, any complexity, whatever phenomena may take place in it (element, apparatus, machine, plant, system). It allows to move toward a better system optimization and a more rational use of plants. Finally it is a logical and coherent stand for energy tariffing.