In this chapter, an interesting approach to the complex topic of transition to turbulence is presented. Flows are classified into canonical flows and complex flows. In Sect. 3.2, free shear flows are presented, and in Sect. 3.3, the Boundary Layer Transition is also presented. Following, still about canonical flows, the transition of Rayleigh–Bénard and Marangoni flows is presented, in Sect. 3.4. It is also presented, in Sect. 3.5, the transition of Taylor Couette flows and, in Sect. 3.6, we talk about Görtler instabilities. To conclude, the transition in complex flows is presented in Sect. 3.7, and, finally, a list of exercises is presented in Sect. 3.8.