Observational Constraints on Black Hole Spin
  The spin of a black hole is an important quantity to study, providing a
window into the processes by which a black hole was born and grew. Further,
spin can be a potent energy source for powering relativistic jets and energetic
particle acceleration. In this review, I describe the techniques currently used
to detect and measure the spins of black holes. It is shown that: (1) Two well
understood techniques, X-ray reflection spectroscopy and thermal continuum
fitting, can be used to measure the spins of black holes that are accreting at
moderate rates. There is a rich set of other electromagnetic techniques
allowing us to extend spin measurements to lower accretion rates. (2) Many
accreting supermassive black holes are found to be rapidly-spinning, although a
population of more slowly spinning black holes emerges at masses above
$M>3\times 10^7\,M_\odot$ as expected from recent structure formation models.
(3) Many accreting stellar-mass black holes in X-ray binary systems are rapidly
spinning and must have been born in this state. (4) The advent of gravitational
wave astronomy has enabled the detection of spin effects in merging binary
black holes. Most of the pre-merger black holes are found to be slowly
spinning, a notable exception being an object that may itself be a merger
product. (5) The stark difference in spins between the black hole X-ray binary
and the binary black hole populations shows that there is a diversity of
formation mechanisms. Given the array of new electromagnetic and gravitational
wave capabilities currently being planned, the future of black hole spin
studies is bright.
