Nitrogenated heterocycles are important motifs that occur in many FDA-approved drugs and they also serve as key building blocks for the construction of natural products, agrochemicals and other bioactive molecules. In the pharmaceutical sector alone, over 84% of approved and structurally unique drugs contain at least one nitrogen atom, with 59% of these displaying at least one nitrogen heterocycle. Among this latter group of compounds, piperidines appear in ~55% of all FDA-approved drugs, making versatile and stereocontrolled methods for their synthesis attractive. Their four-membered azetidine counterparts have also shown promise in medicinal chemistry, but strategies for their preparation have been significantly underexplored compared to 5-and 6-membered N-heterocycles. Given the overall importance of N-heterocycles in bioactive molecules, modular methods to rapidly generate diverse libraries of these compounds with high diastereo-and enantiopurity from readily available precursors are highly sought. Onium ylides are reactive intermediates that contain a negatively charged carbon atom located adjacent to a positively charged heteroatom. Aziridinium ylides comprise a subset of onium ylides which, in contrast to well-studied ammonium ylides, have been underexplored in terms of their utility for the synthesis of complex heterocycles. The reasons for this lack of utility range from unwanted cheletropic extrusion of the ylide nitrogen to regenerate the original alkene, decomposition pathways or the existence of aziridine invertomers that complicate reaction outcomes. However, we were able to successfully achieve ring expansion of simple aziridines to complex N-heterocycles via aziridinium ylide intermediates using four key design features: 1) steric accessibility of the nitrogen lone pair, 2) the presence of an ~sp 3 nitrogen that does not bind irreversibly to the catalyst, 3) the ability to form only one ylide stereoisomer, and 4) restricted pyramidal inversion by employing a bicyclic aziridine. Application of these design principles to carbamate-derived bicyclic aziridines furnished substituted methyleneazetidines and dehydropiperidines (Scheme 1A) through the intermediacy of aziridinium ylides. Despite previous efforts, we have not been able to predictably divert an aziridinium ylide intermediate along multiple pathways to furnish different N-heterocycles from the same precursor. The ability to predictably tune the reaction outcome, while concomitantly installing multiple functional handles in the products for further diversification, would comprise a valuable tool to rapidly access new chemical space with potentially valuable bioactivity. In this work, we highlight how reagent or catalyst control of the addition of methyleneaziridines to transition metalsupported vinyl-containing carbenes can be used to control product outcome. Experimental, mechanistic and computational studies are presented to rationalize our findings and inform future reaction and catalyst design.