SIMBAD references

The status of our understanding of the high-energy extragalactic background from soft X-rays to high-energy gamma-rays is reviewed here. Important improvements were achieved in the measurement of the background flux in some energy bands, yielding a smooth and relatively simple broad-band spectral distribution. Discrete sources are found to contribute significantly to the high-energy background, resolving a large fraction of the diffuse flux at soft X-rays and already about 10% at EGRET energies. Optical identifications and population synthesis models show that a large fraction of the high-energy background is due to active galactic nuclei, QSOs and Seyfert galaxies in the X-ray band and blazars in the gamma-ray regime. However, at the faintest X-ray fluxes a population of normal galaxies with high X-ray luminosities starts to dominate the population. It is argued here, that the majority of these are active galactic nuclei, too.