MIRI is one of four instruments to be built for the James Webb Space Telescope. It provides imaging, coronography and integral field spectroscopy over the 5-28.5um wavelength range. MIRI is the only instrument which is cooked to 7K by a dedicated cooker, much lower than the passively cooled 40K of the rest of JWST, and consists of both an Optical System and a Cooler System. This paper will describe the key features of the overall instrument design and then concentrate on the status of the MIRI Optical System development. The flight model design and manufacture is complete, and final assembly and test of the integrated instrument is now underway. Prior to integration, all of the major subassemblies have undergone individual environmental qualification and performance tests and end-end testing of a flight representative model has been carried out. The paper will provide an overview of results from this testing and describe the current status of the flight model build and the plan for performance verification and ground calibration.