The Galactic Low Mass X-ray Binary GX339-4 was one of the earliest proposed black hole candidates, yet even its most fundamental binary parameters have eluded us for several decades. We present new outburst spectroscopy of GX339-4 which appears to at last reveal the motion of its elusive companion star in fluorescent NIII lines. Based on our multi-site, multi-epoch dataset we propose a new orbital period of 1.7 days and hence derive a mass function for the compact object of 5.8 Msun, suggesting that it is indeed a black hole. In contrast to the moving NIII lines, the wings of HeII are almost stationary, indicating an extreme mass ratio and hence further supporting the identification of the compact object as a black hole. Besides revealing these fundamental parameters, the rich and variable emission line spectrum contains much further information allowing us to dissect the X-ray irradiated accretion structures in this object. We will discuss what we can learn from these data using emission line lightcurves and Doppler tomograms and compare with theoretical expectations and results from similar systems containing accreting neutron stars.