Many realistic astrophysical fluid systems are not composed of gas alone. They are mixtures of gas and such collisionless particles as stars or dusts. These complex systems are realized in galactic disks (mixture of gases and stars), in planetary disks (mixture of gases and dusts), and in others. Furthermore, there are mixtures of gases and photon bubbles. Such mixtures will be realized in optically thick super-Eddington systems. Even in systems of gas alone, there are two-phase systems with spatial separation of two different phases (e.g., HII region and HI region) with their mutual interactions. In this chapter, dynamics of such multi-component or multi-phase systems are addressed. In various extreme physical situations, however, there are other multi-phase or multi-component gaseous systems. They are, for example, two-temperature gases, gases with electron-positron pairs (in high temperature gas), and anisotropic gases (in low density with strong magnetic fields). Two-temperature gases will be briefly mentioned in this chapter, but the gases with electron-positron pairs (Chaps. 10 and 31 ) and anisotropic gases (Chap. 31 ) are not addressed in this chapter.