The population and evolutionary synthesis of the integrated light of clusters and galaxies requires a good knowledge of the underlying stellar spectra. Libraries of observed stellar spectra can be used, but they have several disadvantages, e.g. uneven sampling — which causes problems in the integration phase. Furthermore, no comprehensive library of low– or high–metallicity stars does exist, which would be required to model chemical evolution. Libraries of synthetic spectra could — and should — solve these problems. Using Kurucz and more recent libraries dedicated to cool stars, we constituted a library covering the whole range of Teff: 50 000 – 2500 K.