Nearly all the information we know about the Universe comes from the study of light as it reaches us. The understanding of this information contained in light requires both telescopes capable of resolving light into its different component colours, as well as detailed knowledge of the quantum mechanical behavior of atoms and molecules. This unique book, which is based on a third-year undergraduate course given by the author at University College London, presents the basic atomic and molecular physics necessary to understand and interpret astronomical spectra. It explains what information can be extracted from these spectra and how. Extensive use is made of contemporary astronomical spectral data to both motivate the study of the underlying atomic physics and to illustrate the results.