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We review the electronic properties of pure and doped silicon and carbon clathrates. Using accurate quasiparticle calculations within the GW approximation, we show that undoped clathrates are ~1.8eV band gap semiconducting compounds. Further, the effect of doping by elements more electronegative than Si is shown to lead to p-type doped semiconductors with a ~2.3-2.5eV band gap in the visible energy...