This monograph grew out of a combined effort to prove a conjecture concerning the characterization of Hamiltonian control systems in terms of their variational input-output behaviour. The main concepts and results of this monograph are contained in chapters 1 to 6. Chapter 0 gives a brief introduction to Hamiltonian control systems, with particular emphasis on the relations between physical and control theoretical notions. Indeed, the study of Hamiltonian control systems is one of the places where (theoretical) physics and systems and control theory meet. We conclude the monograph with chapter 7 discussing some possible extensions to the theory presented, as well as some open problems.