Astrophysical objects have many kinds of configurations due to subtle balances among such various forces as gravitational, pressure, rotational, and magnetic ones. Related to this, many kinds of restoring force work on fluid motions when the fluid is perturbed from equilibrium states, which leads to wave motions (oscillations) or instabilities. In this chapter, we explore wave motions resulting from various restoring forces, including shock waves, but those resulting from magnetic and radiation fields are excluded. Wave motions resulting from magnetic fields are described in Chap. 15 , while those from radiation are given in Chap. 25 .