4.1. Early X-ray Surveys
The first large-scale surveys of the X-ray sky were performed by the Uhuru (Gursky & Schwartz 1977) and Ariel-V (Pounds 1979) small satellites in the 2 - 6 keV band in the 1970s. These surveyed the sky more or less uniformly above a flux threshold of ~ 2 × 10-11 ergs cm-2 s-1 and, while non-imaging, were able to derive error boxes small enough to identify many of the sources. One of the early surprises was that approximately half of all the AGN identified by Ariel-V had not been identified previously as active galaxies by radio or optical surveys and had rather different properties (weaker non-thermal continua, narrower weak lines, strong reddening) than optically-selected AGN. The last large solid angle survey in the 2 - 10 keV band was performed by the HEAO-1 satellite, with the largest samples being from the A2 (Piccinotti et al. 1982) and A1 (Remillard et al. 1986) experiments. These surveys produced a list of < 200 AGN and a complete identification of the Piccinotti et al. (1982) list, which had only 35 AGN.