  Significant progress has been made in the last few years on understanding how
supermassive black holes form and grow. In this paper, we begin by reviewing
the spectral signatures of Active Galactic Nuclei (AGN) ranging from radio to
hard X-ray wavelengths. We then describe the most commonly used methods to find
these sources, including optical/UV, radio, infrared and X-ray emission and
optical emission lines. We then describe the main observational properties of
the obscured and unobscured AGN population. Finally, we summarize the cosmic
history of black hole accretion, i.e., when in the history of the Universe
supermassive black holes were getting most of their mass. We finish with a
summary of open questions and a description of planned and future observatories
that are going to help answer them.
