  We briefly review some constraints (Owing to the limited number of pages of
present review, only a sub-sample of the topics discussed during the talk are
briefly summarized. For the interested readers we are pleased to send them upon
request the complete presentation file.) for stellar models in various mass
regimes and evolutionary stages as provided by observational data from
spectroscopy to multi-wavelenghts photometry. The accuracy of present
generation of stellar models can be significantly improved only through an
extensive comparison between theory and observations.
