The interstellar medium — the particulate matter located around and between the stars in a galaxy — occurs in a variety of forms and has temperatures ranging from a few degrees to millions of degrees Kelvin. The properties of the components of the interstellar medium arise from interactions with stars from the period of star formation through stellar death. In this course, we will discuss atomic and molecular processes, along with interactions of radiation and matter and the latest pertinent observations, will be applied toward understanding the physical, ionization, thermal, chemical, emission, and absorption properties of the interstellar medium. Attention will be given to fill regions, planetary nebulae, supernova remnants, cool neutral gas, molecular clouds, hot or X-ray-emitting gas, and particulate “dust” grains. In addition, the global and evolutionary properties of gas and dust in our galaxy will be carefully examined.