The ligands that coordinate the surfaces of UCNPs also play an important role in their optical properties. While UCNP ligand modification has been studied primarily to maintain colloidal stability in various solvents and environments (e.g., in cells), antenna ligands have also been used to dramatically enhance the small absorption cross-sections of UCNPs. Even ligands that do not absorb light can impact the photophysics of luminescent nanocrystals by introducing or suppressing non-radiative pathways. The UCL of UCNPs has been enhanced by small organic ligands, such as picolinic acid, which can alter the electronic structure of lanthanide dopants at the nanocrystal surfaces, demonstrating the potential of ligand engineering for UCNPs. Ligand engineering is preferable to inert shell growth when a small size is needed (e.g., cell imaging and nanoscale devices). However, the high-energy vibrational modes of common organic ligands can quench UCNP luminescence, especially in the absence of passivating inorganic shells.