In this talk, we revise recent results in active control of boundary layer instabilities. First, we analyze the role of the sensor placement with respect to the closed-loop dynamics of the system in terms of stability and robustness. Full-order Linear Quadratic Gaussian (LQG) controllers are used in order to assess the best performance achievable; the trade-off between performance and robustness is discussed by highlighting the main differences which characterize the feedforward control and feedback control strategy. It is shown that in feedforward control configuration, open-loop model reduction is always able to guarantee the same performance obtained by a full-order LQG when convective flows are analysed. In the remainder of the presentation, the delay of transition to turbulence is achieved by quenching three-dimensional perturbations. A setup consisting of localized actuators in the spanwise direction is used.