Transition-metal-catalyzed cycloadditions have been evolved as important and efficient methods to construct cyclic molecules. Among them, transition-metal-catalyzed cycloadditions using carbon monoxide (CO) gas as the 1C synthon provide their unique and powerful approaches to build various cyclic molecules within a carbonyl group, which either acts as an important functional group within the cycloadducts, or can be further elaborated to other functional groups. The tremendous advances in this field are introduced.