In this section, the synthesis of saturated N- and O-heterocycles via formal cycloaddition is presented. The main focus is on metal-catalyzed reactions involving C–C or C–X σ bond cleavage in three- or four-membered rings. After a fast presentation of pioneering works, the important breakthroughs of the last two decades are presented. The section starts with reactions involving three-membered rings. Formal [3+2] cycloadditions of donor–acceptor-substituted cyclopropanes and methylenecyclopropanes with carbonyls and imines are important methods to access tetrahydrofuran and pyrrolidine heterocycles. Formal [3+3] cycloadditions have emerged more recently. On the other hand, reactions of epoxides and aziridines with carbon monoxide or cumulenes are now well-established methods to access heterocycles. These processes have been completed more recently with cycloaddition with olefins, carbonyls, and imines. The section ends with the emerging field of four-membered ring activation for cycloaddition with π systems.