The interstellar medium (ISM) plays an important role in the formation and evolution of a galaxy. The ISM provides the material to form stars and stars in turn inject radiation, metals, and mechanical energy into the ISM, altering the physical conditions, abundances, and distribution of the ISM and affecting future generations of star formation. It is thus essential that we understand the physical structure of the ISM, the physical processes that operate in the ISM, and the interplay between stars and ISM.