In this chapter, we discuss the fundamentals of linear growth of the well-known Richtmyer–Meshkov instability, the Rayleigh–Taylor instability, and the Kelvin–Helmholtz instability, which are instabilities at an interface in fluid. In addition, we discuss the linear growth of the instability of the isobaric slow flame, known as the Darrieus–Landau instability, and that of the instability of the shape of the liquid column, known as the Plateau–Rayleigh instability.