The origin of the X-ray background, in particular at hard (2-10 keV) energies, has been a debated issue for more than 30 years. The Chandra deep fields provide the deepest look at the X-ray sky and are the best dataset to study the X-ray background. We searched the Chandra Deep Field South for X-ray sources with the aid of a dedicated wavelet-based algorithm. We are able to reconstruct the Log N-Log S source distribution in the soft (0.5-2 keV) and hard (2-10 keV) bands down to limiting fluxes

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... f 2x10^-17 erg s^-1 cm^-2 and 2x10^-16 erg s^-1 cm^-2, respectively. These are a factor 5 deeper than previous investigations. We find that the soft relation continues along the extrapolation from higher fluxes, almost completely accounting for the soft X-ray background. On the contrary, the hard distribution shows a flattening below 2x10^-14 erg s^-1 cm^-2. Nevertheless, we can account for >68 the hard X-ray background, with the main uncertainty being the sky flux itself.