Spectral Lines in Radio Astronomy

A review of the elements of microwave spectroscopy as it concerns, or is likely to concern, radio astronomy is presented. This review attempts to show how atomic and molecular resonance lines arise in the radio frequency spectrum and to discuss the important parameters of spectral lines. Finally, it is shown how the intensity of radio emission received from the galaxy is modified by the presence of an interstellar atomic gas having a resonance transition at the frequency of observation.