  The analysis of the properties of the X-ray radiation emitted from
geometrically thin accretion disks around black holes can be a powerful tool to
test General Relativity in the strong field regime. This chapter reviews the
state-of-the-art of gravity tests with black hole X-ray data. So far, most
efforts have been devoted to test the Kerr hypothesis - namely that the
spacetime around astrophysical black holes is described by the Kerr solution -
and X-ray data can currently provide among the most stringent constraints on
possible deviations from the Kerr geometry. As of now, all X-ray analyses are
consistent with the predictions of General Relativity.
