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, that is, 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. 1. Introduction Astrophysical black holes come in a wide range of masses, from ？ for stellar mass black holes [1] to ~ for so-called supermassive black holes [2, 3]. The best evidence for the existence of a supermassive black hole can be found in the center of the Milky Way galaxy, where from dynamical studies, the mass of the Sgr source was established to be ~ [4, 5]. Evidence for the existence of supermassive black holes has also been found in other massive nearby galaxies [6], mostly from resolved stellar and gas kinematics. For active galaxies, it has been possible to use the technique known as reverberation mapping [7–9]. From these observations, a clear correlation has been established between the mass of the central black hole and properties of the host galaxy such as stellar mass in the spheroidal component [10], luminosity [11], velocity dispersion [12, 13] and mass of the dark matter halo [14]. The fact that such correlations exist, even though these components have very different spatial scales, suggests a fundamental relationship between black hole formation and galaxy evolution. Furthermore, it is now well established by simulations [15] that the energy output from the growing central black hole can play a significant role in the star formation history of the host galaxy. In particular, theory suggests that nuclear activity regulates star formation either by removing all the gas [16, 17] or by heating it [18]. It is therefore obvious that a complete study of galaxy evolution requires a comprehensive understanding of black hole growth. Most current black hole formation models tell us that the first black hole seeds formed at . While the exact mechanism for the formation of the first black holes is not currently known, there are several
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