After a basic introduction on how asteroseismology works in practice, we illustrate its application to various kinds of stars of different mass and evolutionary stage. We show how detected oscillation modes allow us to understand details of the interior structure of stars that are impossible to unravel otherwise. The talk will focus on the major questions of how stars rotate internally and what type of chemical mixing they undergo. We highlight as well the asset of asteroseismology as a practical tool for galactic structure and exoplanetary studies. We end by summarizing ongoing and future observational projects.