This module will cover the physics of the interstellar medium (ISM) — the gas in between stars. Students will learn how observations, specifically spectroscopy, allows astronomers to understand the physical conditions and chemical content of the ISM and thereby construct models of the ISM and its relationship to the formation and evolution of stars and galaxies. The module is taught by directed reading and problem-solving. Students are expected to read a section of a textbook and attempt a set of problems every week, and the content and problems will be reviewed at weekly tutorial sessions.