Aimed at senior undergraduate and first-year graduate students in departments of physics and astromy, this textbook gives a systematic treatment of atomic and molecular structure and spectra, together with the effect of weak and strong external electromagnetic fields. Topics chosen are those of interest in astromy, and indeed many were inspired by specific astromical contexts. Examples include the negative ion of hydrogen and the effects of strong magnetic fields such as those occurring on certain white dwarfs and neutron stars. Adiabatic and n-adiabatic handling of electron correlations and application to processes such as dielectronic recombination are included. Astromical examples are provided throughout, as well as end-of-the-chapter problems and exercises. Over seventy illustrative diagrams complete this unique and comprehensive volume.