A modularly expandable electronic control unit comprises: an electronic circuit board on which a conductor track is disposed on a side in a region of lateral edges and enclosing an inner region and separating it from an outer region of the side, a housing having two halves for receiving the electronic circuit board. With the housing assembled, at least one housing half has an encircling electrically conductive shielding wall which rests on and establishes electrical contact with the conductor track and/or has at least one receptacle accessible from outside the assembled housing. The receptacle is disposed so the electronic module placed in the receptacle may be electrically connected to a module connection port disposed in the outer region of the electronic circuit board. The electronic circuit board has electrical connections between the module connection port and one or more electronic assemblies disposed on the inner region.