Commercial solar cells require long-term operational stability. Despite their high performance, perovskite solar cells degrade owing to defects, impurities and mobile ions in the bulk and at the surface of their photo-absorbing 3D metal-halide perovskite films. Compared with 3D perovskites, low-dimensional (LD) perovskites exhibit greater phase stability and superior ambient, light and thermal stability. Notably, by forming 3D/LD heterostructures, these LD layers can also passivate defective 3D perovskite surfaces through surface reconstruction. However, this approach can increase energy mismatch and structural disorder at the contact interfaces owing to excess unbonded ligands. The LD perovskite capping layers can also feature mixed phases, random orientations and other inhomogeneities, which can create charge recombination channels, jeopardize charge transport and undermine long-term stability. Moreover, the monovalent ammonium-based ligands (phenethylammonium and butylammonium) commonly used to create 3D/LD heterojunctions are relatively unstable owing to weak van der Waals interactions btween the organic sheets and the inorganic framework, as well as their relatively low acid dissociation constant (pKa), which make them prone to deprotonation. To improve stability, it is thus imperative to use suitable organic ligands that form strong coordination bonds with the inorganic framework — ideally multivalent amines with high pKa values. Here, we review instability mechanisms at 3D/LD interfaces and discuss mitigation strategies, focusing on ligand chemistry and the fabrication of phase-pure, homogeneous LD capping layers to improve 3D/LD perovskite heterostructure stability.