His research focuses on the formation and evolution of galaxies, with a specialization in the stellar populations of elliptical and lenticular galaxies (ETGs). These systems contain more than half of the stellar mass in the Universe, making them key benchmarks for testing cosmological simulations. He led the development of the MILES stellar population models, an international reference framework that, for the first time, covered the ultraviolet–infrared spectral range at moderately high resolution, based on fully empirical stellar libraries. Its web portal (miles.iac.es) is among the most widely used resources in the field.

Using these models and newly proposed methodologies, this research has broken the fundamental degeneracies affecting stellar population studies, enabling the precise derivation of ages, metallicities, elemental abundances, and the Initial Mass Function (IMF). The results reveal rapid formation timescales (<1 Gyr) in massive galaxies, the non-universality of the IMF, and its spatial and temporal variation. Based on these findings, a two-phase formation scenario has been proposed: an initial in situ phase with an IMF biased toward massive stars, followed by a subsequent evolution toward a more bottom-heavy IMF and accompanied by ex situ growth.