Neutron star (NS) mergers have long been considered ideal sites for rapid neutron capture (r-process) nucleosynthesis. Identifying which elements, and with which yields, are created in NS mergers is key to establishing the extent to which they contribute to Universal r-process elemental abundances. Spectral analysis of kilonovae (KNe), radioactively powered electromagnetic transients associated to NS mergers, offers a robust method to diagnose species created in the ejecta. In this talk, I give an overview of the spectral evolution of KNe as we know it so far, and address the modeling techniques used at each stage. I will particularly emphasise the later epochs of the transient, the nebular phase, which has recently been observed with spectroscopy for the first time by the James Webb Space Telescope.