Reverse engineering in control design proposes practical approaches to build a standard H∞ problem taking into account an initial controller. Such approaches allow to mix various control objectives and to initialize procedures for fixed-structure controller design. They are based on the Observer-Based Realization (OBR) of controllers. The interest of OBR from the controller implementation point of view is detailed and highlighted on academic examples. An Open-soucre toolbox is available to implement these approaches on Matlab