  Accretion onto the massive black hole at the centre of a galaxy can feed
energy and momentum into its surroundings via radiation, winds and jets.
Feedback due to radiation pressure can lock the mass of the black hole onto the
M-sigma relation, and shape the final stellar bulge of the galaxy. Feedback due
to the kinetic power of jets can prevent massive galaxies greatly increasing
their stellar mass, by heating gas which would otherwise cool radiatively. The
mechanisms involved in cosmic feedback are discussed and illustrated with
observations.
