Sep 2012 - Sep 2016, BSc in Physics, Fudan University, Supervisor: Cosimo Bambi.Oct 2016 - Oct 2019, PhD in Astronomy, University of Cambridge, Supervisor: Andrew Fabian.Nov 2019 - Dec 2021, Tsinghua Astrophysics Outstanding Fellow and Shuimu Scholar, Tsinghua University.Feb 2022, Leverhulme and Issac Newton Fellow, University of Cambridge.Black holes are the most extreme objects in the Universe. We cannot directly observe black holes as no light can escape from these light-gobbling monsters. But we can detect the radiation from gas and dust that is being drawn into the black holes through accretion. Unfortunately, due to their large distances, the vicinity of black holes cannot be directly imaged. Instead, we can study the accretion processes onto black holes using spectroscopy, time-series data and other astronomical disciplines. I will summarize recent X-ray observations of accreting black hole systems, including active galactic nuclei and X-ray binaries.