Gas and dust in the interstellar medium

In this article I shall describe the gas and dust in the interstellar medium. I review the evidence for dust and describe its likely nature. Its many roles in the interstellar medium, especially its effect on interstellar chemistry in star-forming regions, are discussed. The nature of the molecular interstellar gas is described, and a brief account of the interstellar chemistry that produces these molecules is given. Several examples of the interaction between gas and dust in star forming regions are given to demonstrate the importance of these components of the Milky Way and other galaxies.