Nearly all information about the Universe comes from the study of light as it reaches us. However, understanding the information contained in this light requires both telescopes capable of resolving it into its component colours and a detailed knowledge of the quantum mechanical behaviour of atoms and molecules. This book, which is based on a third-year undergraduate course taught by the author at University College London, presents the basic atomic and molecular physics necessary to understand and interpret astronomical spectra. It explains how and what kind of information can be extracted from these spectra. Contemporary astronomical spectra are used extensively to study the underlying atomic physics and illustrate the results.