Deep extragalactic X-ray surveys have resolved now up to 90% of the X-ray background, showing that it derives from the integrated emission of a mixture of absorbed and unabsorbed AGNs. Medium Deep Surveys are important to collect a large number of sources for which X-ray and optical spectral analysis are feasible and are more effective than deep surveys to find rare objects, such as type 2 QSOs. I will present the XMM Medium Deep Survey and the logN-logS relationships obtained in the 0.5 - 2 and 2 - 10 keV bands. Next I will focus on the sample of X-ray sources detected with a S/N > 3 in the 2 - 10 keV band, for which optical and infrared data are available. I will present spectral energy distributions and photometric redshifts of these sources and the results of the X-ray spectral analysis. I will also describe 8 type 2 QSO candidates found in the XMDS.