The transition from steady to time-dependent flows is triggered by perturbations. In this lecture the students learn to calculate the growth of small perturbations with a linear stability analysis und they use this method to predict instabilities of flows driven e.g. by temperature or velocity gradients. The students will become familiar with the concept of transition in the absence of linear instabilities. Specifically, they well learn how perturbations can be transiently amplified and can trigger an abrupt transition to turbulence. The students will learn this modern concept by examining simple models, experiments and simulations of flows through straight pipes and in boundary layers. The in-depth analysis of these idealized flow systems will be accompanied by examples of industrial flows.