  In the present work we compare in detail some of the major theoretical
libraries with observations, aiming at detecting weaknesses and strengths from
the stellar population modelling point of view. We compared model predictions
and observations for broad- band colours and for high resolution spectral
features. Concerning the broad-band colours, we measured the stellar colour
given by three recent sets of model atmospheres and flux distributions(Castelli
& Kurucz 2003; Gustafsson et al. 2003; Brott & Hauschildt 2005), and compared
them with a recent UBVRIJHK calibration (Worthey & Lee 2007). We found that the
models can reproduce with reasonable accuracy the stellar colours for a fair
interval in effective temperatures and gravities. The exceptions are: 1) the
U-B colour, where the models are typically redder than the observations, and;
2) the very cool stars in general (V-K > ~3).Concerning the high resolution
features, we measured 35 spectral indices defined in the literature on three
high resolution synthetic libraries (Coelho et al. 2005; Martins et al. 2005;
Munari et al. 2005), and compared them with the observed measurements given by
three empirical libraries (Valdes et al.2004; Sanchez-Blazquez et al. 2006;
Prugniel & Soubiran 2001). We found that the direct comparison between models
and observations is not a simple task, given the uncertainties in parameter
determinations of empirical libraries. Taking that aside, we found that in
general the three libraries present similar behaviours and systematic
deviations. For stars with Teff < 7000K, the library by Coelho et al. (2005) is
the one with best average performance. We detect that lists of atomic and
molecular line opacities still need improvement, specially in the blue region
of thespectrum, and for the cool stars (Teff < ~4500K).
