  The Low Resolution Spectrometer of the MIRI, which forms part of the imager
module, will provide R~100 long-slit and slitless spectroscopy from 5 to 12
micron. The design is optimised for observations of compact sources, such as
exoplanet host stars. We provide here an overview of the design of the LRS, and
its performance as measured during extensive test campaigns, examining in
particular the delivered image quality, dispersion, and resolving power, as
well as spectrophotometric performance, flatfield accuracy and the effects of
fringing. We describe the operational concept of the slitless mode, which is
optimally suited to transit spectroscopy of exoplanet atmospheres. The LRS mode
of the MIRI was found to perform consistently with its requirements and goals.
