Chapter 4 focuses on separated flows that occur in a variety of applications involving external flows, particularly related to aircraft, and internal flows, such as within turbomachines. Flow separation results when the flow does not have sufficient momentum to overcome an adverse pressure gradient, or when viscous dissipation occurs along the flow path. It is almost always associated with some form of aerodynamic penalty, including a loss of lift, an increase in drag, a loss of pressure recovery, and an increase in entropy. This chapter presents both passive and active methods to control these adverse effects.