Chapter 2 is devoted to the temporal stability of incompressible flows. The equations of motion are linearized, and the Rayleigh and Orr–Sommerfeld equations are derived using normal mode analysis. Kelvin–Helmhotlz theory is then introduced for invisicd flows, followed by a number of important theorems related to invisicd flows such as Rayleigh’s Inflection Point Theorem, Fjotroft’s Thoerem and Howard’s Semicircle Theorem, all of which are discussed in detail. The chapter concludes with the stability of the laminar mixing layer.