Spectroscopy of the broad iron iron with ASCA and BeppoSAX has up opened the innermost regions of accreting black hole systems to detailed study. In this contribution, I discuss how observations with future X-ray missions will extend these studies and all us to observationally address issues which are currently only in the realm of the theorists. In particular, high-throughput spectroscopy with XMM and, eventually, Constellation-X will allow the full diagnostic power of iron line variability to be realized. Instabilities of the inner accretion flow, the geometry of the variable X-ray source, and the black hole mass and spin will all be open to study. Eventually, X-ray interferometry will allow direct imaging of the black hole region in nearby active galaxies, thereby providing the ultimate probe of black hole astrophysics.
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