Just like every system, the energy system is a dynamic one, which should match adequately offer and demand sides. Several basic observations are summarized, from which some fundamental trends of the energy system are clarified. The trajectory of the energy system during the next decades will be characterized by both unavoidable trends and by several choices or alternatives Coifurcation points). Major unavoidable trends are: absolute growth of energy consumption, increase of electricity consumption, more emphasis on environmental impacts and on clean coal technologies. On the other hand, many bifurcation points do exist along the trajectory of the futur global energy system. Actions should thus be coordinated in order to manage the trajectory towards a sustainable and efficient energy system.