  The theory of General Relativity has successfully passed a large number of
observational tests without requiring any adjustment from its original version
proposed by Einstein in 1915. The past 8 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 black hole imaging. This is a compact
and pedagogical review on the state-of-the-art of the tests of General
Relativity with black hole X-ray data.
