The Future of Fe-K Line Diagnostics for Probing Strong Gravity

We review what we have learnt with ASCA from studying the Fe-K lines in AGN and describe a program to deconvolve the narrow, non-disk components of the lines with Chandra. This is necessary to derive the correct profiles of the broad, relativistic lines obtained using data from XMM and other high-throughput instruments. Since reverberation techniques are now not looking promising, we present Constellation-X simulations showing an alternative way we might be able to measure black-hole mass and spin.