In “Types of Theoretical Models” we describe two basic types of theoretical models — radiative equilibrium and empirical — that are used to represent stellar chromospheres. The next Section is a summary of recent work on the construction of radiative-equilibrium model atmospheres that show an outward temperature increase in the surface layers. Also, we discuss the chromospheric cooling due to spectral lines. In “Solar Empirical Models” we describe the empirical determination of solar-type chromospheric models that, in order to match observations, imply a temperature rise substantially greater than that predicted by radiative equilibrium. Such a temperature rise must be largely due to mechanical heating. An attempt is made in the concluding Section to apply a scaled solar chromospheric model to a star with a different surface gravity. The results suggest that the chromospheric optical thickness is sensitive to gravity and that the width of chromospheric line emission increases with stellar luminosity, in qualitative agreement with the width-luminosity relationship observed by Wilson and Bappu.