This paper was invited by Pergamon Press to inaugurate its new journal dedicated to Hydrogen Energy. 

The background thesis is that the physical properties of the energy vectors have a preponderant effect on the evolution of energy systems and of the mix of primary energies. Within the frame and the limitations of the arguments presented it appears that hydrogen represents the best energy vector in a broad range of boundary conditions, and that hydrogen-based energy transportation and distribution systems are the most likely to prevail in the long run. 

This kind of analysis is based on some of the results of the IIASA energy group studies on primary energy substitution and transitions.