As described in the previous Chaps. 3 and 4, (cyclo-)passive systems are defined by the existence of a storage function (nonnegative in case of passivity) satisfying the dissipation inequality with respect to the supply rate s(u, y) = uTy. In contrast, port-Hamiltonian systems, the topic of the current chapter are endowed with the property of (cyclo-)passivity as a consequence of their system formulation. In fact, port-Hamiltonian systems arise from first principles physical modeling. They are defined in terms of a Hamiltonian function together with two geometric structures (corresponding, respectively, to power-conserving interconnection and energy dissipation), which are such that the Hamiltonian function automatically satisfies the dissipation inequality.