  The growth of black holes and the formation and evolution of galaxies appear
to be linked at such a fundamental level that we think of the two as
`co-evolving.' Recent observations show that this co-evolution may be complex
and the result of several different pathways. While it is clear that black hole
accretion is linked to specific phases of the evolution of the host galaxy, the
impact of the energy liberated by the black hole on the evolutionary trajectory
of the host by feedback is less clear. In this contribution, I review the
motivations for co-evolution, the current state of the observational picture,
and some challenges by black hole feedback.
