  We discuss constraints on black hole spin and spin-related astrophysics as
derived from X-ray spectroscopy. After a brief discussion about the robustness
with which X-ray spectroscopy can be used to probe strong gravity, we summarize
how these techniques can constrain black hole spin. In particular, we highlight
XMM-Newton studies of the Seyfert galaxy MCG-6-30-15 and the stellar-mass black
hole GX339-4. The broad X-ray iron line profile, together with reasonable and
general astrophysical assumptions, allow a non-rotating black hole to be
rejected in both of these sources. If we make the stronger assertion of no
emission from within the innermost stable circular orbit, the MCG-6-30-15 data
constrain the dimensionless spin parameter to be a>0.93. Furthermore, these
XMM-Newton data are already providing evidence for exotic spin-related
astrophysics in the central regions of this object. We conclude with a
discussion of the impact that Constellation-X will have on the study of strong
gravity and black hole spin.
