  The spin of a black hole retains the memory of how the black hole grew, and
can be a potent source of energy for powering relativistic jets. To understand
the diagnostic power and astrophysical significance of black hole spin,
however, we must first devise observational methods for measuring spin. Here, I
describe the current state of black hole spin measurements, highlighting the
progress made by X-ray astronomers, as well as the current excitement of
gravitational wave and radio astronomy based techniques. Today's spin
measurements are already constraining models for the growth of supermassive
black holes and giving new insights into the dynamics of stellar core-collapse,
as well as hinting at the physics of relativistic jet production. Future X-ray,
radio, and gravitational wave observatories will transform black hole spin into
a precision tool for astrophysics and test fundamental theories of gravity.
