This paper aims at describing (broad lines) the various instrumental concepts for nulling interferometry and how some observational constraints have driven the evolution from the initial Bracewell configuration to the ESA-Darwin mission configuration. Before this short overview, basic principles of interferometry and of nulling interferometry are recalled. Since a key point in the nulling mode is to insert achromatic phase shifts between interfering waves, a short description of several Achromatic Phase Shifters concepts is given as well as the associated constraints.