Understanding the formation and evolution of galaxies is a central topic in modern cosmology and astrophysics. It has been recognised early on that the study of stellar populations is a powerful tool to address galaxy formation, because stellar evolution acts on nuclear timescales that are well understood and are independent of the cosmological model. The modelling of stellar populations allows to derive key information on galaxy formation epochs, stellar masses, and the history of star formation, and even on the Dark Energy content of the Universe. In this talk I will review the state-of-art of theoretical modelling of stellar populations, highlighting areas where progress could be made. I will then discuss key examples where the study of stellar populations allowed to get important insights on the formation and evolution of galaxies.