  Fast variability studies of accreting black holes in the Galaxy offer us a
unique opportunity to measure the spins of black holes and test the
strong-field behavior of general relativity. In this review, I summarize the
arguments often used in attempts of measuring the spins of black holes,
concentrating on their theoretical foundations. I also argue that X-ray studies
of accreting black holes will be able to provide in the future strong
constraints on deviations from general relativity in the strong-field regime.
