This paper elaborates the system engineering methods that are being successfully employed within the European Consortium (EC) to deliver the Optical System of the Mid Infa-Red Instrument (MIRI) to the James Webb Space Telescope (JWST). The EC is a Consortium of 21 institutes located in 10 European countries and, at instrument level, it works in a 50/50 partnership with JPL who are providing the instrument cooler, software and detector systems. The paper will describe how the system engineering approach has been based upon proven principles used in the space industry but applied in a tailored way that best accommodates the differences in international practices and standards with a primary aim of ensuring a cost-effective solution which supports all science requirements for the mission. The paper will recall how the system engineering has been managed from the definition of the system requirements in early phase B, through the successful Critical Design Review at the end of phase C and up to the test and flight build activities that are presently in progress. Communication and coordination approaches will also be discussed.