  Stellar population (SP) models are an essential tool to understand the
observations of galaxies and clusters. One of the main ingredients of a SP
model is a library of stellar spectra, and both empirical and theoretical
libraries can been used for this purpose. Here I will start by giving a short
overview of the pros and cons of using theoretical libraries, i.e. model stars,
to produce our galaxy models. Then I will address the question on how
theoretical libraries can be used to model stellar populations, in particular
to explore the effect of $\alpha$-enhancement on spectral observables.
