The Interstellar Medium (ISM) refers to the matter that exists in the space between stars in a galaxy. It is composed of gas, dust, and cosmic rays, playing a crucial role in the formation of stars and planetary systems. The ISM is primarily made up of hydrogen and helium, with trace amounts of heavier elements. Understanding the ISM is essential for astronomers as it influences the evolution of galaxies. It acts as a reservoir of material that can collapse under gravity to form new stars, while also being affected by stellar winds and supernova explosions, which can enrich it with heavier elements.