The laminar flow in a straight pipe of circular cross section results in an elegant solution in the form of a parabolic velocity profile. Many physicists and engineers have verified this velocity profile and the associated Hagen–Poiseuille pressure drop law. When the flow rate increases with a low level of disturbance at the pipe inlet, the flow maintains laminar, even at large velocities. However, when disturbances are present or artificially added, a sudden transition to turbulence is observed. In this presentation, we will describe the effect of various disturbances and the emergence of turbulence in pipe flow. Specifically, the effect of a synthetic jet, i.e. periodic injection through a small orifice in the wall will be analysed. The effects of the amplitude and the pulsation of the jet on the amplitude stability curve will be presented and discussed. In a second part of the presentation, we will describe the mechanisms of transition to turbulence in a pipe with an abrupt widening.