We describe the hard x-ray mission MIRAX - jointly proposed by teams from Brazil, the USA, Germany and the Netherlands. The scientific objective is to provide continuous 2-200 keV imaging of the central 1000 square degrees of our Galaxy for 9 months per year over up to 5 years. Durign times when the sun crosses the Galactic Center other areas like the Cygnus-, Vela- and the Magellanic Cloud-regions can be observed. MIRAX will detect, localize, identify and study sources of medium to hard x-ray emission, with special emphasis on short-lived, rare and unpredictable events, including weak x-ray transients and fast x-ray movae. MIRAX will reach in a one day observation a sensitivity of 1mCrab in 2-10 keV and 2.5mCrab in 10-100keV. MIRAX will provide a unique capability to study compact galactic objects - notably accreting neutron stars and black holes. It will:- Probe neutron star and x-ray burst theory wiht 20,000 type I x-ray bursts and 50 'superbursts' - Measure spin frequencies of neutron stars from 10-100 burst oscillations - Observe explosive flares and x-ray light curves during ejections in superluminal jets - Study soft gamma-ray repeaters, fast x-ray novae and new types of phenomena yet to be discovered. We describe the science and the instrumentation.