Abstract: Einstein’s theory of general relativity was proposed over 100 years ago and has successfully passed a large number of observational tests in the weak field regime. However, the strong field regime is largely unexplored, and there are many modified and alternative theories that have the same predictions as Einstein’s gravity for weak fields and present deviations when gravity becomes strong. X-ray reflection spectroscopy is becoming a powerful tool for testing the strong gravity region around astrophysical black holes with electromagnetic radiation. In this talk I will present some constraints from the analysis of X-ray data of the black holes in 1H0707-495, Ark564, and GX339-4.