The Interstellar Medium, Expanding Nebulae and Triggered Star FormationThis brief brings together the theoretical aspects of star formation and ionized regions with the most up to date simulations and observations. Beginning with the basic theory of star formation, the physics of expanding HII regions is reviewed in detail and a discussion on how a massive star can give birth to tens or hundreds of other stars follows. The theoretical description of star formation is shown in simplified and state of the art numerical