The study of the interstellar medium incorporates a large range of physical processes on both large and small scales. At the microscopic level, the structure and energy balance of the media can only be understood in relation to atomic and molecular interactions; specifically, two-body chemical reactions and gas-solid interactions. The interaction of electromagnetic waves with atoms, molecules, and solids is also clearly of fundamental importance, since this is how we obtain our information about these environments. On a macroscopic level, gas dynamics is required in order to describe the motions of interstellar gases, the existence of shock fronts, and the consequences of stellar winds and explosions. The aim of this lecture is to provide a comprehensive overview and grounding in the study of the interstellar medium.