The Cosmic X-ray background was discovered by Giacconi et al. (1962) in a rocket flight originally designed to detect X-ray emission from the Moon. The data showed a stong Galactic X-ray source (Sco-X1) and diffuse emission of approximately constant intensity from all directions. The first all-sky X-ray survey with Uhuru and Ariel V in the 1970's revealed a high degree of CXRB isotropy leading researchers to the conclusion that the CXRB has to be mainly extragalactic.
Deep Chandra, XMM-Newton and ROSAT surveys (Giacconi et al. 2001, Brandt et al. 2001, Hasinger et al. 1998, 2001) have shown that the cosmic X-ray background (XRB) is largely due to the accretion onto supermassive black holes, integrated over the cosmic time. The soft X-ray background is dominated by X-ray and optically unobscured AGNs (Quasars, Sy1). The charateristic hard spectrum of the XRB can be explained if most of the AGN are heavily absorbed (type-2). An increasing fraction of highly obscured (intrinsic absorption N_H=10**(21-24) cm**(-2) AGNs are found in the recent hard X-ray surveys. Among these sources is a significant population of heavily obscured type-2 Quasars (some of them already seen with ROSAT.