  The theory of General Relativity has successfully passed a large number of
observational tests. The theory has been extensively tested in the weak-field
regime with experiments in the Solar System and observations of binary pulsars.
The past five years have seen significant advancements in the study of the
strong-field regime, which can now be tested with gravitational waves, X-ray
data, and mm Very Long Baseline Interferometry observations. Here I summarize
the state-of-the-art of the tests of General Relativity with black hole X-ray
data, discussing its recent progress and future developments.
