A key to understand exoplanets is characterization of their host stars. One of the most powerful tools to characterize stellar properties like effective temperature, surface gravity, and metallicity is spectroscopy based on observations of stellar atmospheres. This chapter describes the stellar parameters that can be derived from a spectrum with examples of well-established methods and theoretical model atmospheres. Combined with photometry and parallax measurements, the outcome of the spectroscopic modelling can be used to derive stellar radii and masses.