ABSTRACT
This chapter discusses the role radio observations have played in defining the properties of the cold atomic and molecular gas in the interstellar medium as well as the cold dust. It provides an overview of the properties of the interstellar medium. The chapter explores the time scale for regions to achieve the equilibrium is short enough that different regions cannot stay out of equilibrium for very long. It also discusses how the spectral line has been used to map the distribution of atomic hydrogen in the Milky Way. An additional correction that is often made is to correct for the Sun's motion relative to its local region of the Milky Way. Astrochemistry Nearly 200 different molecular species have been detected in molecular clouds in the Milky Way. The chapter explains the observed thermal emission at radio wavelengths from cold interstellar dust grains.
