Helioseismology is the study of the solar interior using observations of waves on the Sun's surface. It has done much to improve our understanding of the interior of the Sun, testing the physical inputs used to model stellar interiors and providing a detailed map of the Sun's structure and internal rotation. This in turn has greatly influenced theories of the solar magnetic dynamo. These interior studies thus make a valuable bridge between solar physics and studies of the structure and evolution of other stars. Recent developments include new local techniques for unprecedented studies of subsurface structures and flows in emerging active regions, under sunspots, and even of active regions on the far side of the Sun. These developments hold the possibility of a real understanding of how the interior links to solar magnetic activity in the corona and heliosphere. Finally, studies such as those of the deep solar interior are on the verge of becoming possible for other stars exhibiting similar multimode oscillations.