Waves and oscillations are ubiquitous in nature, when the state of matter is either a liquid or gas. The main restoring forces that act on a gas are (i) gas pressure, (ii) gravity, and (iii) magnetic ﬁelds. A gas in a stable equilibrium, which when disturbed will start oscillating. Depending on the external forces, the resulting characteristic oscillations, or modes, are deﬁned as sound, internal gravity, or Alfven waves. The basic physics of the resulting modes are relatively simpler to understand, however, the properties of the oscillations when two or more forces act simultaneously on a gas have a very complicated dependence on the period and wavelength. In the previous chapter, we discussed diﬀerent types of waves and oscillations, purely from a hydrodynamic point of view. We neglected the eﬀect of magnetic ﬁelds in discussing the various modes. However, in this chapter, we shall consider the eﬀect of magnetic ﬁelds; the eﬀect of rotation will be ignored for most of the modes discussed in this chapter.