<p>Molecular engineering, especially the use of Lewis basic moieties, has become one of the most prevailing strategies to boost the efficiency and stability of perovskite light-emitting diodes. Despite substantial progress, the selection of molecular additives often remains empirical, largely owing to an incomplete understanding of how molecular structure governs their functionality. In this Perspective, we focus on recent progress in unveiling the working mechanisms of molecular additives in perovskite light-emitting diodes, emphasizing their distinct structural requirements compared with those in perovskite photovoltaics. We begin by examining the critical roles of chemical interactions and reactions between additives and perovskite precursors. We then discuss the various effects and functionalities of additives, including defect engineering, crystallization modulation and chemical disorder, from a molecular design point of view. Finally, we outline future directions related to stability and rational molecular design and offer an outlook on extending Lewis basic additives towards perovskite-inspired lead-free and spin-polarized emitters.</p>