The nature of light and how it is affected by gravity is discussed. Einstein's prediction of the deflection of light as it passes near the Sun was verified by observations made during the solar eclipse of 1919. Another prediction was that of gravitational redshift, which occurs when light emitted by a star loses energy in the gravitational field of its source. If all of its energy is lost in this way, a black hole is formed. A star at the end of its evolutionary process can form a black hole if its mass is sufficiently large. Gas accreted from its surroundings by a stellar-mass black hole can become hot enough to emit high-energy radiation, which can be observed in X-ray astronomy."Supermassive" black holes could exist at the centres of galaxies--infrared observations have suggested the presence of one in our own galaxy--while others could exist in "active galaxies", including quasars. (Contains 5 figures and 4 boxes.)