In Chap. 6 we have considered various instabilities which occur in the framework of adiabatic processes. Distinct from such instabilities, there are other kinds of many important instabilities which occur by dissipative processes. There are two ways for classifying these instabilities. One is phenomenological, and the other is physical. In the former phenomenological classification, instabilities are classified by whether their growth is monotonous in idealized situation (secular instability) or oscillatory (overstability). In the latter classification, instabilities are divided by physical processes involved (for example, thermal or viscous). Here we classify, for convenience, instabilities in the former way. That is, we classify them into secular instability and overstability. Instabilities which are former in the idealized situations (secular instability) are described in this chapter (Chap. 7) and the latter (overstability) in the next chapter (Chap. 8). It is noted that by this classification some instabilities whose cause is thermal are described in this chapter, and some others in the next chapter (Chap. 8).