Under-deck cable-stayed bridges are innovative bridge schemes that can lead to lightweight and highly efficient construction. However, the design of slender bridges may be governed by the vibrations under traffic live loads. After a historical review of bridges with under-deck cable-staying systems and the consequent research, the dynamic response of medium- and short-span bridges is analysed. Eccentric traffic loads are shown to increase accelerations considerably, and the second vertical mode is found to be the main contributor to the response. A parametric study shows the strong influence of resonance effects in the maximum accelerations registered on the deck. In addition, under-deck cable-staying systems are found to be more appropriate for medium-span than for short-span bridges from the dynamic viewpoint. Moreover, for medium spans, much more slender decks can be achieved compared with conventional bridges without cable-staying systems. Finally, some design criteria are provided that may be useful for structural designers.