A mechanism thought to occur in clusters of galaxies in which hot gas, lost by the galaxies through supernova explosions and stellar winds, cools by emitting X-rays and sinks on to a massive elliptical galaxy at or near the cluster’s centre. As the gas cools it may form into low-mass stars, enhancing the mass of the giant elliptical. Over 70% of the nearest 50 bright clusters have evidence of cooling flows. Reheating shuts down the flows, probably due to input of energy from the active nucleus of the central galaxy.