Black holes are a common feature of the Universe. They are observed as stellar mass black holes spread throughout galaxies and as supermassive objects in their centres. Observations of stars orbiting close to the centre of our Galaxy provide detailed clear evidence for the presence of a 4 million Solar mass black hole. Gas accreting onto distant supermassive black holes produces the most luminous persistent sources of radiation observed, outshining galaxies as quasars. The energy generated by such displays may even profoundly affect the fate of a galaxy. We briefly review the history of black holes and relativistic astrophysics before exploring the observational evidence for black holes and reviewing current observations including black hole mass and spin. In parallel we outline the general relativistic derivation of the physical properties of black holes relevant to observation. Finally we speculate on future observations and touch on black hole thermodynamics and the extraction of energy from rotating black holes.

Published in 2015 as Chapter 1 of 'General Relativity: The most beautiful of theories: Applications and trends after 100 years', Edited by Carlo Rovelli (pub. De Gruyter)