For over a hundred years, optical spectroscopy has been the main tool to study stellar structure and evolution. Photospheric spectra of the electronic transitions of atoms and ions are used to determine the temperature and elemental abundance. Beyond atomic and ionic lines, only the electronic transitions of a few simple molecules (C2, CN, H2O, TiO, CH, etc.) appear in the optical photospheric spectra. With the recent development of infrared and submm spectroscopy, a wide range of molecules have been observed, specially in cool atmospheres of red giants and brown dwarfs. We also realize that beyond the photosphere, a stellar system consists of chromosphere, corona, and stellar wind. Both young and evolved stars possess extensive circumstellar regions and the atoms, molecules, and solid particles in this environment radiate a wide range of lines and bands observable at infrared and submm wavelengths.