Quantum mechanics is fundamental to our understanding of matter and the way in which atoms and molecules interact with light – i.e. spectroscopy. This course introduces quantum mechanics, starting from its development to explain failures of classical mechanics, through to its application to atoms and its use to understand the spectra of one- and many-electron atoms/ions. It is also shown how electronic configurations give rise (through electrostatic interactions) to terms, which are split by the magnetic coupling of spin and orbital angular momenta into (J) levels, which may be further split into states in a magnetic field.