This contribution focuses on the study of 'cool' sources with surface temperatures in the range of about 500-4000 K. In this temperature range spectra are dominated by strong molecular absorption and the tools of modern chemical physics can be applied to compute the molecular opacities needed to simulate the observed spectral energy distributions. (See Bernath (2005) for an introduction to molecular spectroscopy including line intensities and Bernath (2009) for a recent astronomical review article.)