  We discuss some topical issues related to the Fe K emission lines in AGNs. We
show remarkable agreement between non-contemporaneous ASCA and Chandra grating
data and explain why there has been terrible confusion about the ASCA and
post-ASCA results on the relativistic Fe K lines. We point out that in fact the
number of sources (not the percentage) that have been reported to exhibit
relativistic Fe K lines is now larger than it was in the ASCA era. Thus, the
case for Constellation-X as a probe of strong gravity is even more compelling
than it was a decade ago. One of the primary goals of these studies is to
establish the foundation for future missions to map the spacetime metric around
black holes. A prerequisite first step is to measure the black hole angular
momentum in a robust manner that does not rely on assumptions about the
accreting system. In addition, probing the Fe K lines out to high redshifts
will pave the way for studying the accretion history and evolution of
supermassive black holes. However, we point out some issues that need to be
resolved, pertaining to the spin measurement and to the relativistic Fe K line
emission found from AGN in deep surveys.
